How to calculate new "bits" value?

Question: Every 2016 blocks one needs to calculate new bits value. What is the formula to calculate it?
Comment: Are you asking how the difficulty is calculated, or how bits corresponds to difficulty?  Or both?
Comment: I understand how to convert bits->difficulty, I'm asking how difficulty is adjusted. I'm guessing given timestamp from block n and n+2015, there is a formula for it that also takes into account some specific rounding in order to fir the bits field in a block n+2016.
Comment: OK.  I hope my answer tells you what you wanted to know.  It's kind of long, but I wanted to explain the format as well as the details of how 'bits' is calculated.
Comment: It's interesting that you ask about this just as the rules are about to change.  After Feb 15th 2012, testnet gets easy after 20 minutes of fruitless block searching.
Answer: What does the bits field represent?
First of all, we need to understand what the 'bits' field means.
Bits is in 'compact' format.  This is kind of like a floating point format, but it represents big integers rather than arbitrary real numbers.  The first byte indicates the number of bytes the represented number takes up, and the next one to three bytes give the most significant digits of the number.  If the 2nd byte has a value greater than 127 then the number is interpreted as being negative.
To convert a positive integer to 'compact' format, we:

convert the integer into base 256.
if the first (most significant) digit is greater than 127 (0x7f), prepend a zero digit
the first byte of the 'compact' format is the number of digits in the above base 256 representation, including the prepended zero if it's present
the following three bytes are the first three digits of the above representation.  If less than three digits are present, then one or more of the last bytes of the compact representation will be zero.

Example 1 - Convert 1000 to 'compact' format
For example, to represent 1000 in 'compact' format, we convert to base 256:
1000 = (0x03)*256 + (0xe8)*1

So we have a 2 digit base 256 number:
03 e8

The first digit is not greater than 0x7f, so we don't prepend a zero digit:
03 e8

Then the compact representation becomes:
02 03 e8 00

Example 2 - Convert max target to 'compact' format
The minimum difficulty has a target of 2^(256-32)-1.  Let's represent that in 'compact' format.  First we convert it to base 256:
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff

That's 28 0xff digits.  The first digit is greater than 0x7f, so we prepend a zero digit:
00 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff

Now it's 29 digits long.  hex(29) = 0x1d.  So the 'compact' representation of this is:
1d 00 ff ff

Notice we've lost a lot of 'ff' digits there.  We've only kept 2 bytes of precision, what with the size byte and the prepended zero byte using up two of the four available bytes.  If we were to convert back from 'compact' format to see what number we've actually stored, we get:
ff ff 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

which is in fact the maximum target used by Bitcoin.  This is what a difficulty of 1 sets the block hash target to be.
How is the value of the bits field calculated?
Now that we know what the bits field means, we can look at how its value is decided.  In the official client, the bits value is calculated by function GetNextWorkRequired() in src/main.cpp, which does the following:

if we're working on a block that's a multiple of 2016 (every 2 weeks)

look at the timestamps on the last block, and the block 2015 blocks before it
calculate the difference in these two timestamps
if the difference is greater than 8 weeks, set it to 8 weeks; this prevents the difficulty decreasing by more than a factor of 4
if the difference is less than half a week, set it to half a week; this prevents the difficulty increasing by more than a factor of 4
multiply the difference by the current target (ie. the current bits converted from 'compact' representation to the target it represents)
divide the result by 2 weeks
if the result is greater than the maximum target (2^(256-32)-1), set it to the maximum target
convert the result to 'compact' form, and use that as the new bits value

otherwise (we're working on a block that's NOT a multiple of 2016

if we're on testnet and it's later than 15 Feb 2012

if it's been more than 20 minutes since the last block was found

set bits to its highest possible value, 0x1d00ffff, which represents a difficulty of 1; this is the 'special-min-difficulty rule'

otherwise

set bits to the same as in the last non-special-min-difficulty rule block

otherwise (we're not on testnet, or it's before 15 Feb 2012)

set bits to the same as in the last block
Comment: 2^224 is not what you presented, did you mean 2^224-1? http://www.wolframalpha.com/input/?i=2%5E224+in+hex
Comment: Yes, I did.  Thanks!  I've fixed my answer accordingly.
Comment: One thing to add is that the target never changes +- a factor of 4 per change
Comment: @makerofthings7 that's covered by the "if the difference is greater than 8 weeks, set it to 8 weeks" and the following line.  I expanded those lines to make it clear what they're for.
Comment: Thank you & I see now, I'm still working on my C++ to English translation skills ;)
Answer: Here is the relevant code from main.cpp:
static const int64 nTargetTimespan = 14 * 24 * 60 * 60; // two weeks
static const int64 nTargetSpacing = 10 * 60;
static const int64 nInterval = nTargetTimespan / nTargetSpacing;

...

// Go back by what we want to be 14 days worth of blocks
const CBlockIndex* pindexFirst = pindexLast;
for (int i = 0; pindexFirst && i < nInterval-1; i++)
    pindexFirst = pindexFirst->pprev;
assert(pindexFirst);

// Limit adjustment step
int64 nActualTimespan = pindexLast->GetBlockTime() - pindexFirst->GetBlockTime();
printf(" nActualTimespan = %"PRI64d" before bounds\n", nActualTimespan);
if (nActualTimespan < nTargetTimespan/4)
    nActualTimespan = nTargetTimespan/4;
if (nActualTimespan > nTargetTimespan*4)
    nActualTimespan = nTargetTimespan*4;

// Retarget
CBigNum bnNew;
bnNew.SetCompact(pindexLast->nBits);
bnNew *= nActualTimespan;
bnNew /= nTargetTimespan;

if (bnNew > bnProofOfWorkLimit)
    bnNew = bnProofOfWorkLimit;

...

return bnNew.GetCompact();

So the important steps are:

Translate the bits of block n+2015 to a BigNum target.
Calculate the timespan between blocks n and n+2015 (as an integer number of seconds).
Multiply the old target by the timespan.
Divide the result by the desired timespan 2 weeks = 1209600 seconds. This is integer arithmetic so it's rounded down. The result is the new target.
Convert the target to bits.

The code for the CBigNum class is at bignum.h.
Answer: i use the following python code to convert back and forth between "target" and "bits":
import binascii

def target_int2bits(target):
    # comprehensive explanation here: bitcoin.stackexchange.com/a/2926/2116

    # get in base 256 as a hex string
    target_hex = int2hex(target)

    bits = "00" if (hex2int(target_hex[: 2]) > 127) else ""
    bits += target_hex # append
    bits = hex2bin(bits)
    length = int2bin(len(bits), 1)

    # the bits value could be zero (0x00) so make sure it is at least 3 bytes
    bits += hex2bin("0000")

    # the bits value could be bigger than 3 bytes, so cut it down to size
    bits = bits[: 3]

    return length + bits

def bits2target_int(bits_bytes):
    exp = bin2int(bits_bytes[: 1]) # exponent is the first byte
    mult = bin2int(bits_bytes[1:]) # multiplier is all but the first byte
    return mult * (2 ** (8 * (exp - 3)))

def int2hex(intval):
    hex_str = hex(intval)[2:]
    if hex_str[-1] == "L":
        hex_str = hex_str[: -1]
    if len(hex_str) % 2:
        hex_str = "0" + hex_str
    return hex_str

def hex2int(hex_str):
    return int(hex_str, 16)

def hex2bin(hex_str):
    return binascii.a2b_hex(hex_str)

def int2bin(val, pad_length = False):
    hexval = int2hex(val)
    if pad_length: # specified in bytes
        hexval = hexval.zfill(2 * pad_length)
    return hex2bin(hexval)

def bin2hex(binary):
    # convert raw binary data to a hex string. also accepts ascii chars (0 - 255)
    return binascii.b2a_hex(binary)

def bin2int(binary):
    return hex2int(bin2hex(binary))

Comment: Where is **def bin2int()** block in your code?
Comment: @JamesBond I just added it

How to calculate new "bits" value?


A Real Terminal (Root or No-Root)

Question: I use KUBUNTU on my laptop, but this goes for any Linux distro. I can open a terminal window and type in things like sudo apt-get update, sudo apt-get install packagename. Is there a terminal that can carry out commands like this on Android? It doesn't have to be "sudo", I simply mean carrying out commands. I use the Galaxy S3, but I doubt it really matter for the question I'm asking.
Comment: Did you check the Play Store? There [quite a few](https://play.google.com/store/search?q=terminal&c=apps) [terminal emulators](https://play.google.com/store/search?q=terminal%20emulator&c=app).
Comment: Android does not have the apt package manager
Comment: Yes, but can I do something useful with these emulators with no root? On Ubuntu, you still can.
Comment: Yes, at least [one of them](https://en.wikibooks.org/wiki/Android/Terminal_IDE) supports creating new apps in Java using ssh, rsync, vim, etc. from its terminal command prompt, even if you don't have root.
Comment: You can get `su` and other common GNU commands by rooting your device and installing Busybox (busybox is usually installed while rooting) but you can't get apt since it's a package manager for Debian-based distros. Android is not a linux distro, the only common thing it has  is the linux kernel.
Comment: Termux emulates a full Linux environment, including `apt` as package manager and some interesting utilities.
Answer: Summing up from the comments:
First you need to understand that updating apps works different on Android than it does on a "normal" Linux distro: There is no such thing as "apt". Though Android has its own package manager (listening to the pm command), installs/updates are usually dealt with by services/apps like google-play-store or other alternative-markets. pm would rather be comparable with dpkg on Debian-based systems: it can do local installs (i.e. with the .apk file already present on the device itself: pm install would be the command for that), and a lot of other things.
Apart from that: a terminal app can be compared with a terminal on Linux allright. You get a text-mode "window" where you can execute commands. Most are even similar to those available on Linux, as Android is Linux based: you have things like cd, ls, grep, and more. For details on this, you might wish to take a look at e.g.

Ten basic Android terminal commands you should know (AndroidCentral)
Using the terminal (XDA)
Android Terminal Commands (XDA)
Android Shell Command Reference (Github)
our command-line tag-wiki

A look into our terminal tag-wiki will not hurt either :)
I'd say all terminal apps available for Android should support the above. Some even offer additional features, such as an entire development environment (Terminal IDE), or access to remote servers (JuiceSSH).
Comment: @saloalv that's not even the culprit. But it is explicitely mounted with the `noexec` flag (for security reasons).
Comment: Also worth mentioning, installing busybox adds all the commands "you know and love"
Comment: Right, @saloalv – with the amount of commands depending on the version of Busybox :) Don't forget installing Busybox system-wide usually requires root.
Comment: if it'd be possible to install !system-wide, it'd be great for non-rooted users. Unfortunately it seems that the sdcard partition is fat32, and as such the binary cannot be given execution rights
Answer: There is a terminal app which uses "apt" to install packages. Its awesome and has lots of packages ported for arm devices -- Termux  & Playstore Link
Answer: in present days there is the app "termux" on google play store. it comes with a full unix/linux shell and with apt. anything else should be straight forward then. downside: it does not support items like my Android 4.4.2 based tablet.
Answer: Install Terminal Emulator and Busybox in your Android. Terminal Emulator gives command-line window and Busybox APK gives useful commands that are used in Linux like wget, chmod, chroot and more.
Comment: Can you link the apps?
Answer: Assuming that:

Your Android device is rooted;
The su binary is located at /system/xbin/su;
The shell binary is located at /system/bin/sh; and
Your Android device has Terminal Emulator installed.

If all the 4 conditions above are satisfied, then:

If you want to enter the shell and become root (superuser uid 0) so you can execute commands without uid restrictions, start the Terminal Emulator app and just execute this command:
su -

In order to disconnect from the uid 0 (exit superuser status), press Volume - and D, or type exit and hit Enter
Alternatively, if you want to always start Terminal Emulator as root, then start Terminal Emulator, go to Preferences, scroll down to the SHELL section, tap the Command line option and modify the command so it becomes this:
/system/xbin/su -c "/system/bin/sh -"

After you change the command, exit Terminal Emulator (close the application) and start it again. Notice how Terminal Emulator's shell interface now shows you root@something by default.

If you want to add extra commands to your rooted Android system, install BusyBox.
In case your Android device isn't rooted, you still can use Terminal Emulator in order to execute some basic (user) commands such as blkid, cat, cd, clear, cp, du, grep, id, ln, ls, ping, sleep, uptime et cetera.
Answer: Or you can use Gnu/Root Debian for android (GPlay)
Comment: Do you always buy the factory if you just want to rent a car?

A Real Terminal (Root or No-Root)


What are the legal implications of the director and witnesses of the person who forged the signature?

Question: While my husband was traveling on business out of state, under time pressure he pushed my naive 24 year old son to take a legal document requiring my signature (without my prior knowledge or consent) to give him permission to execute construction flip loan documents on our company's behalf. My son only takes orders and does not direct anything. His father does all company directing. My son per his dad's instructions brought these papers to my husband's sister's house to have her forge my signature because his dad had said that it was "no big deal" and an "insignificant" 1 page document. I found out about it and was able to get my son to email me a copy of the forged document (a really poorly done forgery nothing like my signature). My son also sent an email (likely scripted by his dad) explaining why they had "no time" to get ahold of me for my actual signature or permission. Yet they had taken the time to get another family member's signature to trace and got her permission. So I also have that email stating that they didn't ask me first. My husband's sister did the actual forgery I'm told, but what are the legal ramification of the forgery for my husband who directed it to be done and my son who also knew about it and saw it? I believe my husband knew I had reservations about this flip loan and wanted to bypass getting my permission.
Comment: What country do you reside in?
Comment: USA. I have filed for divorce.
Answer: The contract based on the fraud is voidable and all three are criminals
Your husband and sister are guilty of fraud, your son of accessory to fraud. All three are also guilty of a criminal conspiracy to commit fraud. Punishment varies, for example, in new-south-wales fraud has a maximum penalty of 10 years gaol. These are unlikely to be at the maximum end but you never know.
In addition, people convicted of financial crimes like fraud are usually prohibited from being company directors or managing a company at a senior level.
You are also in an invidious position now that you know - if you benefit from the loan you also become an accessory.
If the fraud is proven, the bank cannot rely on this document so the "construction flip loan" (whatever that is) would need to be undone - probably by repayment immediately. The bank could also sue the criminals for their losses - the fees and interest they would reasonably have expected to make on the loan. The bank would probably insist on involving the police.


How do I get back to shooting mode with the D5100?

Question: Yesterday I got my Nikon D5100. After initial setup, I clicked a picture. All okay. Then to learn, I pressed the 'i' button to view the settings. Now I am not able to come out of this information display on my screen. To take a photo I have to see through the viewfinder. I can't see that on my screen/monitor. I can still take pictures and review them. How do I bring back my screen to see the shooting mode?
Comment: That may sound stupid, but did you check the manual? Just googling it, it says there's a switch to go from viewfinder mode to live view mode. Also, usually, if you press the same button that you originally pressed, it would "cancel" the effect. At least on most UIs I know of...
Answer: You should be able to press the button again to come out of the info display (or it may take two presses). On my D700, pressing the shutter button part way turns the LCD off.
However, if you want to view the "to be taken" photo from the LCD, you want to press the button for Live View (check your owner's manual, as the D700 doesn't have this - my old D90 did -probably labelled "LV") and that will display what you are point at, like a P&S camera.
You'll want to read up on Live View, if that's what you want to do.
Answer: On page 44 on http://www.nikonusa.com/pdf/manuals/dslr/D5100_EN.pdf it says the live view switch can be moved towards the rear of the camera to engage the live view mode. The live view mode will lift the mirror allowing you to frame your shot using the lcd. However, because the mirror is raised you will probably not be able to use the viewfinder while in live view.
Hope this helps.
I do not own any Nikon camera all my advise comes from a quick look at the manual from the provided link
Comment: There's also the [Nikon Digitutor](http://www.nikondigitutor.com/eng/d5100/index.html) for those that hate reading manuals :) I do own a D5100, and can confirm: Press the 'i' button to toggle the info display on and off, press the 'Lv' lever towards you to toggle Live view. Just note that the viewfinder autofocus works better than the live view autofocus in low light, so you may want to switch to the viewfinder (turning off the live view) if you have trouble focusing.
Answer: If the USB cable is attached between the camera & PC, the live view LV lever doesn't engage.  Ensure the cable isn't attached to your PC.


How can you regain a love for learning talmud?

Question: I have had many conversations with people who express a falling out of love with learning talmud as they got older. Perhaps it is due to being in the working world or having so many other responsibilities; whatever the case I would like to know if there are any strategies for helping a person, who is 30 years old or older, and who once enjoyed talmud study but doesn't any longer, once again enjoy learning gemarah? I am looking for sourced answers/approaches only please.
Comment: You're more likely to get some decent answers if you remove the requirements for sources on such an ambiguous topic.
Comment: Machonshilo.org
Answer: R. Shimon Fuerst wrote a pamphlet called Tenu Kavod LaTorah in which he discusses how Talmudic students should learn Gemara. His major thesis is that many students upon leaving yeshiva give up on learning Gemara because the style of learning promoted in yeshivos is not conducive to actually amassing a significant amount of Torah knowledge. He says that when these people realize that they were in yeshiva for 10 years and don't actually know much, and can hardly even "learn a daf Gemara b'peshitus" they simply give up. His solution is to revamp the way Gemara learning is done in yeshiva, with at least 100 (two-sided) pages studied per semester. When people see that they have acquired a significant amount of Torah knowledge they will continue to learn.
The pamphlet is only about 30 pages long and can easily be read in its entirety by someone familiar with Hebrew. Here is one excerpt (p.11) that is representative of the point:

והחילוק יהיה רק זאת שאותן מאה דפים ידעו היטב גם עם הראשונים עם הרשב"א
  והריטב"א וכשיבואו לביתם לבקש איזהו פרנסה אם גם לא יזכרו אלף דפים גמרא
  הדק היטב אבל ידיעת התורה בהרבה ענינים בש"ס יהיה להם בודאי ולא יפסוק
  מחמת זה החשק ללמוד להלן אחר שעזבו את דלתות הישיבה
ולא יהיה עוד כדבר הלזה שאנו רואין היום שאחר שלמדו עשרה שנים בישיבה לא
  ידעו ללמוד בעצמן דף גמרא וזאת רק זאת גרם שאינן פותחין עוד גמרא כל ימי
  חייהן שאינן מרגישיו שום חיות בלימוד גמרא והטעם מחמת שאינם יודעים מאומה
The difference will be only this – that those 100 pages they will know
  well, even with rishonim, with the Rashba and the Ritva, and when they
  come to their houses to seek out a livelihood even if they don't
  remember 1,000 pages of Gemara very well they will certainly at least
  have Torah knowledge in many topics in Shas, and they won't cease on
  account of this their desire to learn later on after they have left
  the doors of the yeshiva.
And there will no longer be something like this, where we see today
  that after learning for 10 years in yeshiva they don't know how to
  learn on their own a page of Gemara. This and only this causes them
  not to open a Gemara again for the rest of their lives, because they
  don't feel any vitality in learning Gemara, the reason being that they
  don't know anything.
Answer: R Yehuda Greenwald wrote a small booklet called "Torah study" under the direction of R Shlomo Wolbe (I have it in French, it is part of this larger book).
In it he writes that learning gemara should be the main focus of any Jew. However it is just as critical to learn Humash, Halakha, Hassidut and Musar - all in just proportions and depending on the individual. He recommends 50-60% of time dedicated to gemara, the rest to other disciplines. And insists on the importance of getting advice from a learned rav that knows the person well.
He explicitly discusses the case of many adults who get discouraged after a few years of learning gemara when they realize they won't become the talmidei chachamim they were hoping to be and don't actually enjoy the learning that much. He recommends avoiding to "take refuge" only in learning humash, hassidut and musar but believes a certain balance is important.
I also heard directly from R Levi Azimov (who oversees 120 batei Habad in the Paris area) that learning gemara only is too dry and that a Jew needs "something to warm his neshama in parallel" (his words), e.g., in-depth humash and hasidut.


What are the differences between tone, note, and pitch?

Question: I was wondering what the differences are between "tone", "note", and "pitch"?
Answer: A pitch is a particular frequency of sound, for example 440 Hz.  Wikipedia goes into a lot of detail about how pitch is subjective, and frequency is objective; the frequency that you think you hear (the pitch) might not be the real frequency at all, due to overtones (see below) and other factors.  You can read that article for more details, though for our purposes the definition as "a particular frequency" is sufficient.
A note is a named pitch. Arbitrarily named, of course, by us humans.  For example, Western music generally refers to the 440 Hz pitch as A, specifically A4.  A note can refer to an occurrence of such a pitch as well.  Playing A4 twice can either be talked about as "playing one note twice" or "playing two notes", depending on the context and how specific you want to be.  Notes that are even multiples of other notes share the same name; for example, 880 Hz (double 440 Hz) is also called A, specifically A5.
As Kos points out in the comments, a note can also carry temporal information.  For example, given the same tempo, a whole note is held twice as long as a half note, which is in turn held twice as long as a quarter note, etc.
When a sound consists of multiple pitches is when things get slightly messy.  If a sound consists of multiple pitches, it could either be multiple notes played at once or a single note with overtones (see my answer here for more information on overtones and harmonics).  You cannot determine the difference purely from the sound alone, you have to look at the actual physical action.  Striking two strings on  guitar will produce two notes; striking one will only produce one note, even if that single note consists of a fundamental frequency plus overtones.
Timbre has to do with the other qualities of the sound.  Only a sound produced electronically can have only one pitch; all other sounds consist of multiple pitches.  The mix of frequencies in a sound results in the timbre.  For example, playing A4 on a guitar will actually result in a sound composed of the following frequencies: 440 Hz, 880 Hz, 1320 Hz, 1760 Hz, etc.  The particular strength, or amplitude, of the frequencies results in the timbre.  One sound might have very little 880 Hz present in it while another has a lot, for example, and we can pick up on that difference.  It's how we can tell that an A4 played on a piano and an A4 played on a guitar sound different.
"Tone" is sometimes informally used as a synonym for "timbre", but more often "tone" is synonymous with "note".  It can basically replace "note" any time you would say "note"; e.g. "playing two tones".  Some would argue that it more commonly refers to a single pitch — e.g. a note is a tone plus overtones, and a tone does not consist of overtones — but unfortunately the usage is inconsistent.  "Tone" can also be synonymous with "step": A half-tone is a half-step, and a whole-tone is a whole-step.  For example, from C to C# is a half step or half tone, whereas from C to D would be a whole step.
I don't like to use the term "tone" because of the possible confusion about its meaning.  It's better to use timbre, pitch, note, or step when appropriate and leave tone out altogether.
Answer: Note: (from "notation"): is a certain frequency that you write or read.
Pitch: is a certain frequency that you sing or play.
Tone: is a certain frequency that you hear.
Timbre: is a certain quality (or color) of the tone, depending on the instrument and how you play/sing the tone.
Answer: In a musical context "Tone" can be the hi-cut filter on an old record player or radio. It can be another word for "Timbre". It can be the British name for a whole-step interval (half-step is a semitone).  It can just mean a single-pitch musical sound - a chord consists of several tones played together.
"Note" could be the same as that last definition of "Tone".  Or it could be one of those marks sitting among the 5 lines of a musical stave.
"Pitch" at least is fairly unambiguous. It refers to the frequency of the note, normally described by musicians by letter-names A,B,C etc, plus sharps and flats.  Or by the vertical position of those "notes" on the 5 lines.
I understand your urge to "get the terms sorted out".  But I don't think it will help much.  Better to explore some music, ask "What does this DO?". "What is this CALLED?" is of secondary importance, and often isn't cut-and-dried anyway.


How to Introduce Uncertainty without Scaring a Customer?

Question: Recognizing uncertainty is fundamental to an Agile mindset. When leading the transition to a more agile mindset, how do you tell your customer that you, as a development team, generally operate in a world of uncertainty, when they have always looked to you to simply "get the job done? "
Comment: Show them that the reason the job hasn't always been done is because of the lack of recognition of uncertainty. It was always there, just not acknowledged.
Comment: Thank you, everyone, for your constructive answers. I went with the approach of not bothering the customer with uncertainty. Instead, focusing on setting the stage for the customer to be exposed to more work in progress and more frequent communication. This approach is gleaned from Charles Cobb's book "Making Sense of Agile Project Management."
Answer: While not being a trained expert on the Agile ways, I would try to not bother the customer with uncertainty in the way of "you might not get what you asked for", but rather give him certainty that something (and be it just confirmation of the viability of a solution) can be done.
I think agile methodologies can help here. Take Scrum. Your team should commit to certain things for the upcoming sprint. You don't want to commit something you might not be able to deliver due to uncertainty. So I would try to phrase the commitment differently, to state that the goal for this sprint is to identify possible solutions for this problem. And another story later is to implement one of them (or some workarounds).
Of course those risky, critical points have to be identified as early as possible in a project, so that the investigation "stories" can be done soon enough (especially if there is a risk that the entire project might fail because of them).
I realize that investigative stories like that might not be proper Scrum stories. But personally, I don't care :)
I believe it's easier to sell a story like that to the customer than tell him that you will do Story A in the next sprint, but it might not work.
Comment: I agree. Make a promise, keep a promise. That way you pay into your "credibility account". And keep communicating with your customer. If one day something goes terribly wrong you are in a much better position. If you make it transparent that you intend to spend some time to decide for one of multiple solutions customers often are understanding and sometimes even offer help.
Answer: I used to use a commuting analogy.
You know it normally takes between about 40/45 minutes to get to work, so there's a basic level of uncertainty there: 40 to 45 minutes, but you don't know whether it'll be 40 minutes, 45 minutes, or somewhere in between. So you allow 45 minutes.
But on a bad day (e.g. when the traffic is unusually heavy, or a train is cancelled), it can take up to an hour. This only happens about once a month, but if you want to be certain you'll arrive on time, you allow an hour.
But maybe once a year there's a horrendous delay (e.g. multiple car pile-up, two trains cancelled and the third one delayed), and it takes an hour and a half. So you have to decide whether you want to allow 90 minutes every day, and get in 45 minutes early most days, or accept that you'll be late every so often.
If you want, you can take this further by drawing a Normal Distribution graph, with percentage bands, explaining that increasing certainty has a cost, and ask him what level of certainty he would find acceptable.
Comment: If it a normal distribution graph represented on-time delivery wouldn't we expect as many projects to be early as there are projects that are late? Equally wouldn't we expect that task/stories/etc. have an equal proportion that is finished in a shorter time and that is finished late? In my experience, no matter how hard we try, any of this is true. I don't have the book right here but I believe Don Reinertsen make a similar point in one of his books.
Comment: @John Yes, we would. Steve McConnell makes the same point in Rapid Application Development.
Comment: The distribution of time to complete a task is certainly not normal - it is skewed heavily with a long tail to the right (as your train example suggests). It is much easier for a task to take twice as long as expected, than to take half as long as expected.
Answer: Thank you for your reference to my book "Making Sense of Agile Project Management".  You're on to something very important, in my opinion.  The level of uncertainty in a project is one of the most important determinants of the project management approach (either agile, traditional, or hybrid).
An example I've used in some of the talks I've given to PMI chapters is an example of building a bridge across a river...it would be ridiculous to take a very agile approach for that kind of project..."let's build the first span, see how that comes out, and then we'll figure out how to finish the rest of the bridge" - that doesn't make any sense at all!
The mistake many agilists make is assuming that the customer doesn't have a clue of what they want and that you need to start from scratch as if there were complete uncertainty and it's futile to develop a plan. That is rarely the case - I've never worked with a customer where there was no expectations about what the project would cost and how long it would take to complete it.  There's generally an expectation that you will do the best you can to develop a plan based on whatever information you have about the project and continue to refine that plan as the project progresses.
In some cases, where the uncertainty is low, you may be able to develop a fairly accurate estimate and plan (e.g. building a bridge).  In other situations where the uncertainty is high, the plan may be very thin and sketchy and subject to a lot more adjustment as the project progresses; and of course, that needs to be mutually understood with the customer.
I have a model I've developed called "Managed Agile Development" that consists of two layers: a plan-driven "macro" layer and an agile "micro" layer…the plan-driven layer can be thick or thin depending on the level of uncertainty in the project.  It has worked great in several situations I've been in.
The key to getting this to work is developing trust with your customer.  If you don't have trust and credibility with the customer, it will be very difficult to get this to work.
I hope that helps...
Chuck Cobb
Comment: +1 Thanks for the answer, Chuck. Nice to see you on the site.
Answer: I think it comes down to selling the benefits of the approach. Using the agile approach, your customers still get what they want, and they start getting some value sooner, and they don't need to have all of the details hammered out before you start. So you are giving them a better experience with delivery starting in a shorter timeframe, and more opportunities on their part to control the scope of the work - and hence the spend - over time.
It might be difficult to put all of this into a proposal to the customers, however, if it hits them "cold". If they are used to getting a fully-costed proposal, and you present them with something that is not what they expect, they are going to be uncomfortable, which is not what you want them to be. So, you really should be thinking about warming them up to your preferred way of thinking, well in advance of producing your next proposal to them. Consider developing a presentation that explains the approach and the benefits (from the customer perspective) and then sitting down with your customers to see what they think of the way ahead. Be prepared to be challenged, and don't expect a rousing success the first time out - but keep the door open for ongoing discussions on the subject. Some ideas take a while to sink in!
If they are still reluctant to accept what you are saying when you have to make your next proposal to them, you still have the option of producing a conventional (some would say old-fashioned) proposal that won't upset them, and you should be able to retain the customers' business. If the feedback from the customers is more positive, and they are happy to  buy in to the Agile approach, you have a win-win result.
Answer: I think it is rare for a customer to cope with the notion of uncertainty in a typical buy-seller relationship.  Most of us to some degree suffer from a serious case of illusion of control in ourselves, wherein we have a tendency to believe--really believe--that we can control our environment.  The notion of random results affecting our own performance is foreign.  Accepting it when we buy services from others is even less appetizing.
For the really savvy buyer with strong business acumen and with a buyer-seller relationship that moves towards a trusted adviser and true partner, then the introduction of uncertainty is possible.  Without these two variables in place, I think most customers would think the seller is trying to make up excuses early to blame uncertainty and the randomness of results for their poor performance.
In most cases, I think the notion of uncertainty is best left with the seller and its risk management processes.  You are left with active mitigation, early communication, great customer service, and hope your customer can tolerate you when work randomly falls to the unfavorable side of things.
Sadly, though, this works against the customer.  This sort of forces sellers to pad their estimates, to push their targets to the right side of the curve, causing the schedule and cost to grow.  While this increases the likelihood of success, the customer pays for every piece of it.
Answer: Make them dream, I mean share your vision for them to see the final result in their imagination. If the customer is uncertain, it is because he thinks you are not able to acheive what you are saying you want to achieve for him.
Simon Boulanger, BAA MBA Bth ACS FLMI


Is a 2-hour layover in Newark fine when traveling to Bombay?

Question: I will be travelling to India from DFW with a connecting flight on United. I have a 2-hours layover in EWR Newark. Do I need to do immigration in Newark or in DFW? Both my flights are with United. What would be my terminal for the Mumbai flight from EWR? Is 2 hours enough?
Answer: Assuming you have purchased this as a single ticket, two hours should be quite sufficient unless your flight is significantly delayed. You won't go through any kind of immigration control departing the US, just normal airport security at DFW and Indian immigration when you arrive at BOM. At EWR, all you'll need to do is get to your next gate and board the plane.
Most United flights at Newark depart from Terminal C, and that seems to include the normal gates for DFW and BOM flights. If you do wind up in Terminal A, there is a shuttle bus between terminals inside the secure area that you can take so you won't have to go through security again.
Answer: Easily enough.
You only need to walk to the right gate, and 15-20 minutes are usually enough if you don't diddle around in shops.
Also, if your incoming flight is late, they will usually wait (as they would be responsible for your overnight stay and rebooking if the connection fails)
Comment: " they will usually wait". That's very unlikely. Just last week I saw United passengers from LAX to both MEL and SIN coming in  basically a few seconds late to the gate and the gate agent refused to let them on even though there was still plenty of time left to aircraft door closing.


BJT gain.Width of base region vs collector current

Question: We know that in active region Ic=beta(Ib),so it means that as Ib decreases the Ic should decrease proportionally,but the early effect states that as we increase Vce the width of the base region decreases hence less recombination ( less base current as less electrons travel into base ,for PNP) therefore the Ic increases..
Aren't these two concepts contradicting or am i missing something ?Explain for common emitter bias.
Please help me clear my doubts.Thanks in advance.
Answer: At first, you have to realize that the emitter current - and accordingly the collector current - is controlled by the base-emitter voltage Vbe (see Shockleys famous exponential equation). I am aware that in some textbooks the BJT is described as a current-controlled device, but that is a simplified description which cannot explain all observable effects. The base current does exist (unfortunately) and it is taken into account for calculating the base bias circuitry - but it is kind of by product.
(Barrie Gilbert: ...the base current is purely incidental - it is best viewed as a "defect").
Now - what happens if the voltage Vce across the BJT increases? This increase results in a broader depletion layer between collector and base (Vcb increases). As a consequence, the space charge region between base and emitter (forward biased by Vbe) decreases. Therefore, the electrical field strength within this area is increased (constant base-emitter voltage) - and more charged carriers are able to move to the collector. This effect somewhat increases the collector current (and reduces the base current for a constant emitter current because of Vbe=const.).
Note: This happens even in the case that the base current is kept constant (Ib=const.). Hence, this is one of some proofs that it is the voltage Vbe which controls Ic.
Comment: You say that the base current decreases but collector current increases ,doen't that defy the relation of Ic=beta(Ib)?
Comment: No - because B (resp. beta) are no real constant factors. Both parameters depend somewhat on Ic as well as on Vce. But because of the large tolereances the actual values are, normally, unknown - and we treat these factors as if they were constant.
Comment: so you mean that more(less) recombination in the base region is actually decreasing(increasing) alpha (in case of common base) rather than changing Ib ?!?!
Comment: i have one more query, does more recombination in base region mean more base current value in the circuit (for compensation of  the electrons which have recombined with the holes)?that means that base current is increasing but the collector current is decreasing (due to less holes from emitter reaching the collector)?this is my main doubt here!


Is registering for an invention fair considered as public disclosure?

Question: if a person sends a summary of an invention to an invention fair for primary registration before patenting the invention, this will be considered as a subject of public disclosure?
 If it is so, it will be considered as prior science or it is patentable yet (For PCT or US patent)?
Comment: Any public disclosure is public disclosure. I wouldn't disclose to anyone at all without a signed non-disclosure agreement in place before file a patent application.
Comment: thank you @Eric-Shain, As grace period is 12 months in US law, I thought it is the same for PCT or other countries too. And I'm not sure what is the condition for PCT
Comment: I don't believe there is a universal 12 month grace period in Europe, but I'm not an expert. At the very least consider filing a provisional patent in the US to establish a priority date earlier than any potential publication. I believe you can use that priority date in later PCTs.
Answer: The PCT does not have any "grace period", it leaves that to each Contracting State and the national patent law thereof. If the International Searching Authority finds a disclosure made by the inventor prior to the international filing date, it will add a note either in the Search Report itself or in the Written Opinion. Once you enter national/regional phases, the corresponding national/regional patent offices will use that disclosure against the patentability of your invention if there is no grace period under the corresponding patent law.
You have a summary of national/regional grace periods in this document of the WIPO.
I do not know what that registration you mention entails, and whether it qualifies as public disclosure or not, but I am pretty sure that in Europe it would be prior art that can be cited against your invention. At the EPO you have Art. 55 EPC (most Contracting States of the EPC have something similar in their national patent laws) concerning "non-prejudicial disclosures". According to that Article, an invention displayed by the inventors "at an official, or officially recognised, international exhibition falling within the terms of the Convention on international exhibitions" can be a "non-prejudicial disclosure" if it occurred not earlier than 6 months before the filing date of the European patent application. There are at least two important things to note here:

There are very few exhibitions that are accepted for non-prejudicial disclosures. In other words, almost every exhibition is not an exhibition that qualifies for such non-prejudicial disclosures.
The Article mentions that the invention is displayed, so only the displaying of the invention at the exhibition can be made non-prejudicial. The registration that you mention is not the same as displaying the invention, thus it does not qualify as a non-prejudicial disclosure (having an NDA in place, as @Eric Shain indicates, may reduce the likelihood that said registration will become prior art).

As a sidenote, in order to make the exhibition of the invention non-prejudicial, there are many steps that need be taken. There is paperwork that needs to be requested from the organization that makes the exhibition, said paperwork must be issued in a particular time period, and then it must be filed with the EPO. Missing the timeline in any of the steps that need be taken causes the exhibition to be prior art.
I do not know much about these legal aspects in the US, so that part will have to be answered by somebody else.
Answer: In the U.S. something becomes prior art under 35 USC section 102

(1) the claimed invention was patented, described in a printed
  publication, or in public use, on sale, or otherwise available to the  public before the effective filing date of the claimed invention; . .
  .

Sending in the registration would not likely be considered a publication but if/when is printed in a guide to the fair it would be a printed publication - even if only online. Would it be considered available to the public? That is a good question for the patent bar exam so I do not recommend counting on it not being "know to the public".
Of course it depends on the level of detail you put in the disclosure. If you only explain the result or purpose and do not describe the structure that achieves that result you have not disclosed the invention. The U.S. does have an odd 1 year grace period. As another answer mentions you can file a provisional patent application with the USPTO. One idea is to file the exact same thing you use for the fair registration as the text of the provisional.
Comment: Thanks a lot but, at 35 USC section 102 section b it has described the exceptions. "(b) EXCEPTIONS.—
(1) DISCLOSURES MADE 1 YEAR OR LESS BEFORE THE EFFECTIVE FILING DATE OF THE CLAIMED INVENTION.—A disclosure made 1 year or less before the effective filing date of a claimed invention shall not be prior art to the claimed invention under subsection (a)(1) if—
(A) the disclosure was made by the inventor or joint inventor or by another who obtained the subject matter disclosed directly or indirectly from the inventor or a joint inventor; or..."
Comment: that's why I mentioned there was an odd 1 year grace period. IT is odd because the USPTO has in interpreted it to cover only the exact disclosure not being prior art.


two complex analysis multiple choice questions

Question: True/false:

There is a bijective analytic function from the complex plane to the upper half-plane.
There is a non-constant bounded analytic function on $\mathbb{C}\setminus\{0\}$.

Please help anyone to solve these problems.
Comment: What are your thoughts on them?
Answer: Hints :
For $1$, suppose that $f: \mathbb{C} \rightarrow \mathbb{H}$ is holomorphic, where $\mathbb{H}$ is the upper half-plane. Thus $\operatorname{Im}f(z) > 0$ for all $z \in \mathbb{C}$. Now consider $g(z):=e^{if(z)}$. Then $g$ is entire, and $|g(z)|<1$ for all $z \in \mathbb{C}$ (why?). Now what can you conclude about $g$, and then about $f$?
For $2$, Suppose that $f$ is analytic and bounded in $\mathbb{C} \setminus \{0\}$. Is it possible to extend $f$ to bounded analytic function in $\mathbb{C}$? (Hint : Riemann removable singularity theorem). Now what can you say about $f$?


How does the 180 day Schengen rule reset?

Question: I entered Sweden July 17 from the US. My 90 days were up on Oct. 14 however I applied for a tourist extension to extend 30 more days. I never got a response and the consulate told me I was okay to stay in the country until I got a response (even though I was there longer than 30 extra days). I ended up leaving Nov. 20.
The 180 days from July 17 would be Jan.12 2019. I stayed 120+ days of the 180 legally. Does a new  clock rest on Jan.12? I would like to re-enter Schengen country on Jan.13
Answer: There is no "resetting", and the particular 180-day period that happened to start on the day you first entered has no special significance.
The rule is: On any given day you can only use the "short visit" rules if you have been inside the Schengen area on at most 90 of the last 180 days.
If you were to attempt to enter on January 27, for example, the relevant question would be:

how many days have you been in the Schengen area (other than as authorized by a long-stay visa or residence permit) in the 180 days from August 1 to January 27?

The answer is "112 days already", and if you were allowed to enter on January 27, it would become 113, which is more than 90. So you cannot enter as a "short visit" on January 27.
After a continuous stay of 90 days or more, what this works out to is that you need to be outside the area for 90 entire days before the "number of days inside in the last 180 days" has become small enough that you can enter again. If you left on November 20, the earliest you can enter will be February 19.
Comment: @phoog: Possibly, or "forgot which months have 31 days" errors.
Comment: Day counts in this answer were done in my head and may be subject to off-by-one errors or worse.
Comment: Or worse...like off-by-two errors?
Comment: Nice one.  It's a shame that this rule is so difficult to explain.
Answer: As already answered there is no reset on new year or something like that. However not all days that you spent in schengen count for the calculations of the schengen visa. I do not know what exactly a tourist extension is but it may not count in your calculations.
I know this because In the case of a friend she had been in Germany on a student visa and when it ran out she didn't have her permanent visa yet and would have been more than 90 of the last 180 days in Germany (160). So she inquired and it turns out that only the days that you were in schengen using the schengen Visum count, so she had 0 of the last 180 days used.
Comment: The Borders Code excludes _"periods of stay authorised under a residence permit or a long-stay visa"_ from the count -- but whatever the OP means by a "tourist extension" it sounds clear that they were never _issued_ with any document, so they can't have had either a residence permit or a long-stay visa.


Discussion about Academia Community Polls

Question: Please discuss any ideas and issues about Academia Community Polls in this thread to keep it tidy.
Comment: I have been told in direct response to my poll that only demographic questions are appropriate in this community poll. Is this the intention of the [Academia Community Poll](http://meta.academia.stackexchange.com/questions/467/academia-community-polls?) ? Can someone please clarify this in the rules section.
Answer: Two recent poll questions have been added by @user8005.
Do you agree that evolution is the best explanation for the origin of human life on earth? and Do you think there's a purpose of life?.
I am not sure what the purpose of the poll questions are, but these seem so far off topic that I think they should be deleted.
Comment: The two questions have been deleted.
Comment: @DanielEShub I don't check meta very often. I would not answer user8005's question on the main site and chat room if I had seen this comment earlier. Obviously, he is not someone I would like to be talking to. Thanks for alerting it.
Answer: The phrasing in "Are you currently working in Academia (i.e., employed by a university with a primary duty of performing research)?" is overly narrow in excluding anyone whose primary duty isn't research.  This amounts to saying many faculty members aren't academics at all, for example those in liberal arts colleges, teaching-focused comprehensive universities, community colleges, etc., as well as many adjuncts or teaching faculty in research universities.
I'm not sure how to fix this, given that we don't know whether the answers so far are based on the narrow interpretation or not, but it's worth keeping in mind in the future to avoid causing offense to academics who aren't primarily researchers.
Answer: Regarding the question "Are you currently working in Academia (i.e., employed by a university with a primary duty of performing research)?"
I'm not sure how a student should answer this one.
If s/he gets the salary form the university? If s/he is on fellowship?
Comment: I think all students would answer yes, independent of their funding.
Comment: I would expect all students to answer "no" because they are students foremost. That is, I read the question in a way that "working" did not include students. Perhaps this is because I did not think of myself as "working" even when I had a graduate assistantship ...
Comment: Didn't think about this. I agree with @Daniel, I would definitely say "yes"; your primary duty is research. This also implies that non-research masters students (i.e., professional masters) would answer "no".
Answer: The polling format does not let people change their answers when their status changes.
The following questions for example have answers that vary in time:
Which of the following "roles" describes you best?
How many publications in peer-reviewed journals have you made in your career?
Are you currently working in Academia
This one, without going into the details of how "happy" is an ill-defined descriptor, probably has an answer that varies frequently:
Are/were you happy in your PhD program?
People can up-vote another answer, duplicating entry, but not cancel their previous votes.
Comment: Given the limitations of the SO platform for this kind of pollings, should we be really interested in their results, we can think of resetting the results once a year.
Answer: Well, since we are a site of academians here is my curmudgeonly take on polls like these. Feel free to upvote/downvote or discuss.
Problems with interpreting polls like these

It isn't a random sample of the population of academics, so inferences about the distribution of responses for Academics in general can not be made, under any reasonable circumstances.
Even questions that focus specifically on the population that participate here on the Academia site, it isn't a representative sample, so inferences to even the characteristics of individuals who use the site are very questionable.

It is fairly simple to illustrate the problematic aspects. Those more active on the sight are more likely to participate in meta. Thus non-response in polls is not a random sample of the individuals on the site. Some might argue this is a good thing (if you are more active you should be given more weight), but to suggest this results in any reasonable inference is tantamount to saying two wrongs make a right.
This doesn't even cover other problematic aspects of voting on the comments that may prejudice the results, such as comments that are placed first have higher exposure, and after accumulation of so many comments some are collapsed from the view. Both these circumstances will be likely to bias polls to already up-voted answers, making long term collection of the polls potentially even more sytematically biased.
I don't deny such poll questions can be fun and intriging, but lets be serious about what useful information that can be gleaned from such things. Potentially more useful ways to carry out some of the analysis that motivated the poll question are to utilize the Stack Exchange data explorer (when we get out of beta), or to just scrape info. from user pages. I realize some of the questions are not possible from here, but that isn't a reason to pretend like comment polls are a reasonable solution.
For instance, in regards to whether we are CS centric, it wouldn't be trivial to scrape the data, but if you just go to user pages you could look at the reputation on other sites. You could then look at the distribution of users here to determine if people from either of the computer science sites post a disproportionate number of the questions/answers here. In terms of location data that would be pretty simple when we are out of beta (see a similar question and answers on the stats site).
(For fairness these are both problematic as well, for people don't need to connect there accounts and people don't need to supply location information, but again, problems with these approaches don't justify innapropriate polling!)

To place the distinction between these polls and generally interpreting upvotes/downvotes anywhere on the StackExchange sites, the (typical) goal is to ask a question and receive an answer. Votes (and checking as answered) are generally taken as tokens of the correctness/usefulness of the answer to the original question to outsiders.
While upvotes/downvotes on any question are similarly suspect to critiques of making inferences, any answer can be evaluated in absentia other context by a reader and determine its usefulness. That is, the poll of up and down voting is not the goal of asking a question, and if the site blinded them tomorrow the questions and answers currently on the site would not suffer from it.
The question that these polls attempt to measure is merely what proportions of the community conform to certain categories. Usefulness, should (I hope) be measured in how accurate an estimate they provide. Here, the upvotes and downvotes are the answer, and I've already provided reasoning as to why they aren't likely to be a very good answer to the question.
Comment: I don't understand. Are you arguing that the poll is biased because it's not a uniform sample (so my votes count too much), or that it isn't weighted by response frequency (so my votes count too little)?  Are you perhaps mistaking a poll on a web site for a scientific instrument?
Comment: There sure are many biases, but I think: 1. it's better than nothing (data and user page scraping has many (more?) issues); 2. it gives a good feeling of the equilibrium *of the heavily involved part of the community*… if we keep that in mind, it is a useful tool.
Comment: In regards to population. The comments I was concerned about where "are the people who vote to close questions US/CS centric". So I think a meta poll probably samples the correct population. I don't know enough about the comment sort order to know how it affects bias, but I am sure it does.
Comment: @F'x I don't see how data and user pages *has more* issues. I can understand the sentiment that it is *better than nothing*, but lets interpret for what it is. Polling *en masse* will not be a reasonable tool to make inferences about the population, and can potentially be misleading. Scraping data from user pages has a direct advantage in that you can tie it to actual responses. For example, Jeffe's vote can only count once in a user poll - but he answers so many questions that any question asking about user contribution to the site would be interested in such info.
Comment: @DanielE.Shub - I don't even see how your meta question addresses that question - you ask for *everyones* field and *academic systems you are familiar with*. People voting on these aren't necessarily the people voting to close questions! All of these polls are merely an guess that they capture some reasonable information from the population.
Comment: @JeffE, it is a bit of cognitive dissonance to admit the poll isn't a reasonable instrument to measure user characteristics of the site and then forge ahead anyway pretending it has some other utility. I don't view answering questions on here any differently than I do giving colleagues or clients advice. I sure wouldn't condone such a poll in any other circumstance, so why should I pretend like it is ok here?
Answer: The time zone poll got a bit confused, because it appears that some people answered based on daylight saving time (summer time) for their time zone, whereas others answered based on standard time.
Answer: Doing  little clean up of the poll page. There was a question regarding this comment/answer
Academia Community Polls
Which adds a non-binary gender type. I think it is covered at http://gender.wikia.com/wiki/Non-binary
Comment: I think the gender question should just be deleted altogether instead of trying very hard to make it politically correct. It's completely irrelevant anyways.


Can we add a 'split answer' feature?

Question: It's really annoying when someone answers a question like 'what is your fave tool?' and they write 4 in a bulleted list. This makes it hard to comment on the individual bits. It would be cool if you could drag over one of the answers, hit the 'split to new answer' button and have it become its own item.
Comment: Why? If another answer is also valid, it will get plenty of upvotes. Accepting one as answer is basically just another upvote with a slightly bigger value.
Comment: You should add a comment to the poster to indicate which part you disagree with, and they might change that part if they agree with you.
Answer: Since this kind of question is tolerated at max, but definitely not encouraged, I don't think there's much to be done.
Also, this sounds close-to-impossible to reasonably implement UI-wise. If you have strong feelings about a particular case, you can post a new answer with one part of the source answer, and edit the source answer to remove the migrated part.
Comment: In that case you would get the reputation for the new question and not the original answerer.
Comment: @Eric: Questions like this should be wiki basically *by definition*. There's no rep involved.
Comment: @balpha: Good point, but it's still questionable.  Badges can be awarded for wiki questions, and people are generally against answer 'stealing' and this could be viewed as such even if the intentions were honest.
Comment: From the FAQ: "Like Wikipedia, this site is collaboratively edited. If you are not comfortable with the idea of your questions and answers being edited by other trusted users, this may not be the site for you."
Comment: @balpha: Editing someone's answer to improve it and editing someone's answer to remove content which you have posted, after the fact, as a different answer are separate things. The former is encouraged. The latter is, in my opinion, even worse than just copying and pasting someone's answer.
Answer: Since the Trilogy strongly discourages discussion type or list type questions, although to some degree some exists, this is really not a needed feature. Comment to the OP and ask them to break up the answer, and since these questions are meant to be CW, they won't have any rep gain either way.
I don't see a need for this. Anything to discourage users to post discussion or open ended questions I would welcome with open arms.
As balpha pointed out it will also be a pain to implement. What logic do you use? Split by line? Split by word?

Can we add a 'split answer' feature?


Is Canon 50mm f/1.2 with Canon EOS 80D suitable for portraits, landscapes, travel/nightlife photography?

Question: I'm purchasing my first DSLR camera (Canon 80D) in a few weeks. I want to use it mostly for portraits, landscapes, travel/nightlife shots. I've come across the Canon EF 50mm f/1.2L, and wanted to know if it's a good lens for those kinds of shots.
Comment: Why not try the 1.4 Version for starters? It's only a fraction of the price
Comment: Or even the 2.8. The included kit lens is always a good starter point, then you explore other options to fit your informed needs. Faster lens are nice (#bokeh), but are also tricky to use and might not behave as you expect them to: https://photo.stackexchange.com/questions/105445/how-to-make-pictures-lots-of-kids-so-everyones-face-isnt-blurry . An expensive tool is probably a well build tool, but not necessarily the _right_ tool for the job.
Comment: I think it is an excellent lens for portrait, travel and low light photography. It is usually the only one I bring when I go sightseeing. Not so good landscape though, and 1.2 seems overkill, both in price and weight.
Comment: Is that an aspherical lens? If you want to use it for night photography, that makes quite a difference in how it will behave wide open...
Comment: Have you considered mirrorless?
Answer: Beyond the fact that any 50mm is probably too long for a single lens solution on an APS-C camera for landscape and travel photography:
The EF 50mm f/1.2 L is designed for a very specific purpose: taking wide aperture portraits. That's about all it is really optimized to do very well. Some of the design decisions made to achieve this goal makes it unsuitable for tasks that even much less expensive 50mm prime lenses can do better.
For example, the modest EF 50mm f/1.4 is probably a better lens to use for landscapes at medium apertures of around f/8. So is the EF 50mm f/1.8 STM! If you can find one, the discontinued EF 50mm f/2.5 Compact Macro would be much better suited for doing art or document reproduction work for flat objects (or for shooting flat test charts at short distances).
This is because the simple double Gauss design upon which the f/1.2 L is based leaves much of the lens' field curvature uncorrected. This causes the lens to render a curved field of focus. It also allows the lens to render very smooth out of focus areas, particularly out of focus highlights. That's where this lens shines and why many portraitists are willing to pay the asking price for it.
Even though it is a fairly expensive lens, that does not mean it is "better" than any other less expensive 50mm lens for every single purpose to which one might want to use a 50mm lens. In this case, it means it is a specialized tool that many seasoned photographers have trouble learning how to use effectively.
If the EF 50mm f/1.2 L were a car, it would be an exotic sports car with a non-synchronized manual shift transmission and very unforgiving "race tuned" suspension. It's great for going very fast in the hands of an expert at the track. But a Toyota Corolla is better suited to driving in stop-and-go city traffic.
Comment: It's been fairly well documented by multiple reviewers/testors that the 50/1.2 is not a good astro lens for precisely that reason.
Comment: "8 elements, 6 groups, including one glass molded (GMo) aspherical element.

The lens has a floating design with which the lens adjusts itself to optimize correction as it's focused." (Ken Rockwell) ... talking about two different lenses, or NOT "simple double gauss"?
Comment: You are correct, it's not a simple six-element double Gauss (Gauss is a proper name). But even with the floating aspherical lens at the very back, [the design acts very much like one](https://global.canon/ja/c-museum/wp-content/uploads/2015/05/ef392-lens-construction2.gif). The first three elements are pretty much the same as the first three elements of a double Gauss. The next four element in two groups fulfill the same function of the fourth and fifth elements. The role of the floating aspherical element at the rear is to correct for chromatic aberration at all focus distances.
Comment: Compare the block diagram above to [this double Gauss block diagram](https://upload.wikimedia.org/wikipedia/en/8/8b/Double_gauss.png). See also [Designing a Double-Gauss lens, the hard way](https://www.slideshare.net/operacrazy/designing-a-doublegauss-lens-the-hard-way)
Comment: See also Roger Cicala's lens rentals blog entry [Lens Genealogy Part 1](https://www.lensrentals.com/blog/2011/08/lens-geneology-part-1/) where he lists the Canon 50/1.2L as a modern double Gauss design.
Comment: I don't have this lens myself, I do wonder if it would have the same problems specific to night photography (macroscopic coma on point light sources) that actual "simple double gauss" f1.2 designs tend to have....
Answer: I'm using a Canon 80D aswell, bought the 50mm 1.8 and found it way to narrow almost every time. I recently got the sigma 18-35mm 1.8 and it's my go to lens now. I would advise you to buy cheaper lenses first to figure out what you want and need. The Canon 24mm 2.8 is another really nice (and sharp) lens which is awesome because of its size.
There are so many good options out there and a variety of cheaper lenses is in my eyes much better than one really expensive one (especially with your first DSLR).
Comment: +1 for the EFS 24mm 2.8, awesome and cheap lens. 1k$ + lens are for people that know _exactly_ what they want and need. Unless you have money to burn (In which case I'm not sure you would be buying a 80D... did you see the new a7R III?)
Answer: If you have to ask if it's the right lens for you, it probably isn't.  I know a lot of tutorial articles and videos on youtube will talk up a 50mm prime like it's the greatest thing ever, but they're generally talking about on a full frame sensor, and even then, it is very limiting if it's your only lens.
You certainly will have a very tough time with landscapes and travel shots if that's all you have in your bag.
The 80D comes with a choice of kit lenses, and either would be a better option for your needs, at least to start.  The 18-135 is pretty fantastic imo, autofocus with it performs great and it covers a focal range that will satisfy almost all of your needs and is very well suited to travel photography.
Answer: The characteristic likely to cause problems is the focal length. Consider using an inexpensive Nifty Fifty (50/1.8) to determine whether the focal length will work for you. A "normal" lens, 28-35mm on crop sensor, will tend to be more useful.
With respect to a 50/1.2 lens on a crop-sensor body:

Portraits. It should work well. Images will look similar to those taken with a 80/2 lens on a FF camera.
Landscapes. Probably will not work too well. A wide-normal lens would work better. (18-28mm)
Travel/Nightlife. It could work. But a normal lens (28-35mm) would likely be more useful. Depends on your personal preferences.
Answer: It'll work, but it won't be the most flexible combination and it will limit your compositions, especially for travel. There will be travel shots you just can't get at 50mm on an APS-C sensor: wide angle compositions when in tight streets near buildings, for example.
The 50mm f/1.2 is incredibly expensive when you don't really need f/1.2. You would be better off with the "nifty fifty" f/1.8 and spending the money you save on the 24-105mm f/4 L. It's lighter and cheaper than other options like the 24-70 f/2.8, and image quality is excellent. And when you need razor sharpness wide open pop on the nifty fifty.
Comment: There's not *that* much weight difference between the EF 24-70mm f/2.8 II (805g) and the EF 24-105mm f/4 L IS II (795g) or EF 24-105mm f/4 L IS (670g).
Comment: I had the original 24-105 f/4 L IS :-) The 24-70 f2.8 is 135g heavier (~5oz for our US friends). That'd save about the same weight as the 50mm f1.8 :-)
Comment: 80D w/battery + EF 24-105mm f/4 L is 1,400g. 80D w/battery + EF 24-70mm f/2.8 L II is 1,535g. That's less than 10%. Even the original EF 24-70mm f/2.8 is equal to or sharper than the EF 24-105mm f/4 L IS all the way from 24-70mm at the same apertures. The EF 24-70mm f/2.8 L II is sharper than the original. The 'Nifty 50' is *almost* as sharp at 50mm as the EF 24-70mm f/2.8 II when using common apertures. At f/5.6 and narrower there's no real difference between any of these lenses. The other disadvantage of the 24-105 is that it is T5.1 for most FLs. The EF 24-70mm f/2.8 L II is T2.9.
Answer: On a Canon APS-C camera, a 50mm lens will become the full-frame equivalent of an 80mm lens, which is a mild tele-photo. 85mm is a classic portrait lens for face and head-and-shoulder shots, and the 50mm f1.2 on your 80D will certainly excel at this.  The lens could also be useful for certain landscape shots, though I find a longer telephoto, such as a 200mm (on a full-frame body), to be more useful for isolating bits of the landscape.
Of course, what you find the best lens for certain things is subjective, but I'd struggle to use an 80mm equivalent lens on it's own, especially for travel shots. However, combine it with a wide angle lens, you now have a very versatile kit. I worked quite happily with a full-frame camera and just a 28mm and a 85mm lens when I started out and rarely wanted for anything else. However, as far as I can see there is no real APS-C equivalent of a 28mm in the Canon line-up.
If you are after a single lens solution, then the most versatile prime lens, in my opinion, is the 35mm equivalent lens — although the 35mm is not great for portraiture.  On a Canon APS-C camera that would equate to a 22mm lens. The closest thing in the canon line-up is, I think, the 20mm f2.8, which I'm told is a great lens and is modestly priced too.
Comment: `However, as far as I can see there is no real APS-C equivalent of a 28mm in the Canon line-up.` - A lot of zoom lenses get that focal length, but you are right that there are no primes in the 28/1.6 = ~18mm range.
Answer: This is sn EXCELLENT lens
For everything
Weaknesses for the purposes you listed.
Travel & landscapes
its heavier and costlier than 50/1.8
You may wish to have wider or mor tele quite often thus a one lenses does all 18-300 solves this.
You wont need the f/1.2 often except in very dim light. Digital has much higher useable iso than film did
Nightlife
Its expensive to bring to nightclubs having it scratched or bumped the 50/1.8 is slower but dtill fast in this digital era
Portraits.
Not much tele, youll have to get close and you wont have much out of focus separation from subject to background
You would get better results with more tele
Starting at 85/1.8, 105/2, 135/2, 180/2.8 300/2.8
200/4
Often you wont be shooting wide open
F/1.2 was the dream lens in film. Today, it is rarely needed except low light where flash isnt allowed.
It is a fantastic single fixed lens
It lacks the versatility of teo zooms or a single mega zoom or for the money, 3 lenses
16-35 (18-55)
50/1.8
100-400 (70-200, 70-300,)
Comment: How is this both "an excellent lens for everything" _and_ "lacks the versatility..." of others? Your answer seems at odds with itself.
Comment: There are also a lot of typos, weird capitalization, and punctuation errors in this answer — I assume you are doing this from your cell phone. It would be nice to others if you could go back and straighten that stuff out.


Was Harry Potter the first magic series to use wands?

Question: Was Harry Potter the first magic series to use wands? I can't really think of any other series that used them.
Answer: NO!
Wands have been associated with magic for millennia, both in fiction and in the real-world practice of both stage magic (conjuring tricks for an audience) and occultism (purported real magic). You can learn more about wands and their history in association with magic at sources such as Wikipedia and Esoteric Archives.
In reality
The earliest recorded use of wands by supposed magicians appears to have been by the magi of ancient Persia, priests of a religion which is at least two or three thousand years old. You can read here about the use of a "barsom" or "baresman" - a stick or bundle of sticks, essentially a wand - in prehistoric Zoroastrian religious rituals. Here's a quote from that page and a 2500-year-old image of a magus with a wand:

According to Kotwal and Boyd, the baresman is an "ancient Indo-Iranian emblem of seeking the Holy", and it "establishes a connecting link between this getig [material] world and the menog [spiritual] realm. The barsom is, as it were, the conduit through which the archetypal principles and powers manifest their presence and receive the offerings." (A Persian offering, 1991, p. 6, 10; words in square brackets are mine). It is also an instrument through which one acquires the sacred power (op.cit. p. 23). Perhaps then it is also a conduit for channeling the power outwards, and thus is a prototypical 'magic wand'.

The concept of using wands to practise magic extends through ancient Greek and Roman times to medieval Europe.

It's mentioned in the 800-year-old Sworn Book of Honorius, one of the oldest and most influential texts of medieval European magic. The full text of this book is available online in both the original Latin and an English translation. You can search for "wand" on that page to see how it appears; the original Latin word is virgula meaning "a small rod, stick, wand, or staff".
There's also more detail about wands in the 500-year-old Key of Solomon, another textbook on magic. Again the full text is available online, and there's an entire chapter entitled "Of the knifes, sword, quill knife, iron pen, short lance, wand, [staff,] and other instruments of magical art". You can see the wand ("virga") at the bottom of this illustration:

In fiction
The earliest known use of wands in written fictional literature is in the 2800-year-old Homerian Odyssey. In book 10, Odysseus and his crew encounter the sorceress Circe who uses a magic wand to turn the men into pigs. From this translation:

She brought them in and made them sit on chairs and seats, and made for them a potion of cheese and barley meal and yellow honey with Pramnian wine; but in the food she mixed baneful drugs, that they might utterly forget their native land. Now when she had given them the potion, and they had drunk it off, then she presently smote them with her wand, and penned them in the sties. And they had the heads, and voice, and bristles, and shape of swine, but their minds remained unchanged even as before.

A more recent and rather well-known example from fantasy literature is the 70-year-old series The Chronicles of Narnia. In the (first published, second chronologically) book The Lion, the Witch, and the Wardrobe, the White Witch Jadis uses a golden wand to perform magic such as turning creatures into stone. For example:

Edmund saw the Witch bite her lips so that a drop of blood
  appeared on her white cheek. Then she raised her wand. "Oh, don't,
  don't, please don't," shouted Edmund, but even while he was shouting
  she had waved her wand and instantly where the merry party had
  been there were only statues of creatures (one with its stone fork fixed
  forever half-way to its stone mouth) seated round a stone table on
  which there were stone plates and a stone plum pudding.

TL:DR: there are literally hundreds, if not thousands, of books featuring magic wands which predate the popular Harry Potter series: both textbooks of 'real' magic and entertaining fictional stories, dating from thousands of years ago up to the present day.
Comment: And if you extend wands to include staffs, that includes classics like the Wizard of Earthsea or Lord of the Rings. The tradition of a wooden stick in magic is rather broad =)
Comment: @user14111 *You* haven't read a classic fantasy book but have seen a movie based on it? I'm floored.
Comment: If I remember right the blue fairy in *Pinocchio* — I mean the Disney movie, I haven't read the book — used a magic wand. But that's not a series.
Answer: I have heard it argued that the staves of Moses and his brother Aaron were magic staves as a religious miracle, such as Moses parting of the red sea, and magic are one and the same; the only difference is the view of the reader.
Comment: See also https://scifi.stackexchange.com/q/71209/4918 "What was the first SFF work where magic (spells) had to be primarily channeled through wands?" on Sci Fi SE, as well as https://scifi.stackexchange.com/q/193004/4918  https://scifi.stackexchange.com/q/100667/4918
Comment: Would these staves count as "wands"?
Comment: I view a magic staff as just a bigger form of a magic wand
Comment: What about the "rods" that Aaron and the Pharaoh's magicians turned into serpents? Were those magic wands?
Comment: The staves of Moses and Aaron are known by either name, the staff/rod of Moses, the staff/rod of Aaron; as such I would class the rod and the staff as big magic wands.
Or magic wands are smaller versions of the magic rod and/or magic staff.


Electric start isn't functioning anymore

Question: The electric start of my motorcycle isn't working anymore. I can hear a buzzing noise. I can still start it using kick start though.
Can it be fixed?
Please help!
Comment: What model and year Yamaha do you have?
Answer: There can be two things that are preventing your motorcycle from starting.

A dead Battery: Check if your Horns,lights and other electrical systems are working when the vehicle is not turned on(do this in the morning before starting your motorcycle) or at least the horn should work.If the intensity of the horn sound dips after a second or two then it's probably your battery, in that case you will have to check for your battery plus dynamo/rectifier with the mechanic or an ammeter.
Dead Starter motor: Your starter motor might be bust, if it is an old vehicle then there are chances that your starter motor might be dead or needs servicing.
Answer: It sounds like your battery is going bad
The buzzing or rapid clicking sound you hear is your starter solenoid.  When you hit the start button on your handlebar it sends voltage to your solenoid.  The solenoid has a small electromagnet that flips a switch to send battery power to your starter.  If the solenoid is buzzing then that would mean that your battery is not holding a strong enough charge to keep the solenoid powered enough to maintain contact for the starter circuit.
Possible Issues
The issues you may be experience include:

Battery won't hold a charge, it's bad and needs replacement
Stator issues.  You might have a bad stator that isn't generating any power.
Rectifier.  You have rectifier/voltage regulator that isn't charging the battery and is bad.  As well, it might not be converting the generated power from your stator to DC current from AC

If I had to guess and just swap out a part.  I would swap out the battery and see if that rectifies the problem.  If not, come back here and ask how you can test out your system to identify what the root issue is.
Here are some helpful links.

How to test a motorcycle charging system
How to use a multimeter
Answer: I think, that starter motor, but firstly You should check the voltage on power connections on starter, when the start button is turned ON. Secondly, try to clean all connection around starter, a "+" connection and the ground. If it's doesn't help, plug out starter, unscrew, pull out and look inside, maybe electric brushes are over. One more thinks.. if the voltage on connections will be zero find and check relay!
Best!
Answer: I Have just sorted a problem with my Zonte Tiger 50cc starter was clicking not turning engine over. I had a fully charged new battery had starter motor out and cleaned it the bike is only 2 years old.
After thinking again I removed flywheel cover and found a lot of rubbish inside so cleaned all around with WD40 and removed spark plug the engine turned over lovely. This for info for anyone whol has starter problems with engine not turning so dont just think it;s the starter motor.


Multiple Search Service Applications

Question: I see that it is possible to create multiple Search Service Applications. How can you get a Search Centre to use one Search Service Application over another.
Answer: I found the option :)
Central Administration >  Application Management > Service Applications > Configure service application associations
You can change the service applications for a Web Application here.

Multiple Search Service Applications


What is the best way to show that a link opens a modal window?

Question: I want to make the user feel that clicking the item will bring up more detail about it in a modal window.
Comment: For one, don't use a standard link style like you use for your other links. And possibly don't use a link at all.. but like a round button with a question mark.
Comment: @LowerClassOverflowian I would love it if you give me link(s) to example(s)
Answer: Modal windows are quite common nowadays so the user might not but surprised to see something like that appear.
The content of the modal window is more important than the pattern itself.

If the content is an action, it is a common pattern to use
ellipsis (aka suspension points).

If the content is just information and long, you should just use a simple link or a button.

Clicking this button will open a modal window (it says : About the playlist)

If the content is just information and short : do not use a modal window, use a tooltip.
Answer: The best way would be have an icon accompany the URL. Off the top of my head, I know of two icons that lend themselves to representing a modal dialog:
External Link

New Window

You could then have a tooltip over the icon that says something to the effect of:
View more details in a modal dialog

Icons source: jQuery UI ThemeRoller
Comment: Isn't a modal dialog supposed to be contained in the current window? An external link should open a new window.
Comment: @n.1 - Of course. I'm not saying otherwise. I'm simply saying those 2 icons visually lend themselves to represent a modal dialog as well.
Comment: Oh.  I was going to ask "what is the icon for modal dialog?" :)
Answer: I don't know about the general convention but I had the same problem and used the following solution.
The link is displayed as a bootstrap button and I added the fullscreen glyphicon.
Here is the result:

First icon is edit and second icon is fullscreen, I hope the user will understand that by clicking on the link he will be able to edit the entity in fullscreen, in this case a modal window.
Answer: Having call to action whether it is a link or a button with "View more details" as label for example should do, modal windows are pretty ubiquitous these days, do you have any reason why you want to explicitly let the user know that clicking this will open a modal window?
This could be perceived as an unnecessary warning. If your user want to see more info, let him click and open your modal window.. no big deal :)
Comment: It's actually a table item. Additional information is impossible in my case.
Comment: -1. See THAT question RIGHT THERE is actually designing for the users experience! Actually mattering about the user and thinking a step ahead vor him (preparing him for what might come). I would definitely **not** *let him click* and surprise him.
Answer: Unfortunately there is no fixed standard to show that whether a link will leave the page or remain on it.
However, some websites use a link which shows a dashed blue underline when it is hovered.  This may help convince your users that the action will not leave the page.
You could also provide the user with more information without taking up any screen space.  Just set the title attribute of the element.  This text will appear in a tooltip when the user hovers over the link.  So you can explain to cautious users what will happen when they click.
CSS:
a.modal-link {
  text-decoration: none;             /* Suppress the default underline */
}
a.modal-link:hover {
  border-bottom: 1px dashed #88f;    /* Show a light blue dashed line on hover. */
}

HTML:
Details


Is IT Management on or off topic?

Question: The area 51 IT Management proposal was floated at about the same time as this one, but hasn't yet made it out of the "define" phase. Based on the discussions here, the IT Management community feels their management issues are different enough.  For them PM is a tiny subset of IT Management, although frankly, all of their reasons appear to smell a lot like project management to me.  That said, the poor guy who floated the merge question got down voted a lot.
Answer: Is it a bad thing to try it out?  Instead of asking if it's okay, I'd suggest just trying a few questions to see how it goes.  If the questions seem too "off-topic" then we can close them.
The best way to determine if they'll fit pm.stackexchange is to just give it a shot.
The nice thing about StackExchange is that tags can be used to organize questions.  If IT Management questions are tagged as "IT Management", then this could help organize the content and separate the questions for users not interested in IT Management while still unifying the site for users who are interested in both.
Comment: +1 that it fits and no-harm in trying it out at this point. Make sure its clear that its about managing IT projects vs managing IT infrastructure/hardware itself : )
Comment: Perhaps the tags should be software-management, hardware-management and it-infrastructure-management to avoid confusion.
Comment: @DannyVarod - This is an old question. Since this time, we've modified the [faq] to assert that any problems related to PM is on topic. We have a software development tag, but no tag for IT Management. I believe IT management questions would be off topic here as a general rule, unless it was a PM question about an IT project. In other words, a PM IT question would be on topic here, but not every IT management  problem is a project management problem. I'm almost certain we've had PM questions about IT projects here. The question is: Are these projects so different that they actually need a tag?

Is IT Management on or off topic?


What was Beverly Crusher's exact position at Starfleet Medical? What do we learn regarding this character`s absent year?

Question: During the second season of Star Trek: The Next Generation, what exactly was Dr. Beverly Crusher's position within Starfleet Medical and what do we know regarding her accomplishments or chosen area of medical research/application during this time?
Answer: From "The Child":

WESLEY: It's going to be hard leaving the Enterprise.
PICARD: Mixed feelings for all of us. It's always difficult leaving any ship, just as it was for your mother when she left to become head of Starfleet Medical.

She left to become head of Starfleet Medical. We don't know much about how she spent her time there, what research she pursued, or anything of that sort, but one can presume it was mainly administrative duties. We do know that she at least briefly worked with Commander Hutchinson:
From "Starship Mine":

HUTCH: Beverly. Lovely as ever. How do you manage it?
CRUSHER: It's good to see you again.
HUTCH: It's been far too long. What is it, four years? Now I want to hear   everything that happened after you left Starfleet Medical.
Comment: Thx. I always felt a little lack of closure with regard to her absence S2.  Like a plot hole regarding a fav character.  But those excerpts really help.  I agree with you that she was most prob stuck with administrative stuff, but thanks especially for the 2nd exerpt.  I had no recollection of that exchange, but it does give room to assume that she created and enjoyed professional relationships that, in part, most prob made for a fruitful year.
Comment: Isn't head of Starfleet Medical an Admiral's position?
Comment: That depends on what your definition of 'is', is, @LordPraxis. Seeing as 1) Beverly isn't an admiral, and 2) Beverly as the head of Starfleet Medical, clearly it's not requisite.
Comment: Daniel B - If Beverly was the head of Starfleet's entire medical department, as the plain phrase "Starfleet Medical" indicates, she must have been an admiral during the period that she was head of Starfleet Medical.  I find it much more normal for a commander to be a temporary admiral while having a job that is an admiral's position, than for the head of Starfleet's entire medical branch to have a lower rank than admiral.


What is a Reverse Proxy?

Question: I know what a proxy is, but I'm not sure what a reverse proxy is.  It seems to me that it's probably akin to a load balancer.  Is that correct?
Comment: http://stackoverflow.com/questions/224664/difference-between-proxy-server-and-reverse-proxy-server
Answer: A reverse proxy, also known as an "inbound" proxy is a server that receives requests from the Internet and forwards (proxies) them to a small set of servers, usually located on an internal network and not directly accessible from outside. It's "reverse", because a traditional ("outbound") proxy receives requests from a small set of clients on an internal network and forwards them to the Internet.
A reverse proxy can be used to allow authenticated users access to an intranet even when they are located outside. Users on the internal network can access intranet servers directly (their IP address is their authentication), but users outside it must authenticate themselves to the proxy server (usually with a username and password) in order to be allowed in.
Comment: The name 'inbound proxy' would have made better sense to me when I first heard about reverse proxy.
Answer: Yes correct, a reverse proxy is very often used for load balancing. A reverse proxy presents a single interface for multiples servers and distributes requests between those servers. Its most common uses are load balancing and caching.
Answer: A reverse proxy can be used to provide security for internal resources. Two common ones are Microsoft ISA server, and Apache with mod_proxy.
As an example, if you have a Microsoft Exchange server in your internal LAN, but want to give access to internet users for Outlook Web Access (OWA), you could place your server in the DMZ (NOT RECOMMENDED), or open ports in the firewall to your internal network. With a reverse proxy, you would put the reverse proxy in your DMZ, and all OWA requests are directed to the reverse proxy. It then takes that request and forwards it to the Exchange server, acting as a middle-man.
With Apache mod_proxy and named virtual hosts, you can reverse proxy for multiple sites with a single IP address, depending on whether SSL is involved.
This way your servers and data are protected, while still allowing secure access.
Answer: Reverse Proxies will also often be used for caching expensive-to-compute resources; this lets you generate a given page one time per minute (for instance) instead of once per request. For high-traffic sites, this can be an important benefit, particularly for the home page; many sites see 70% of their traffic go to the home page and no further. A caching reverse proxy can serve up a static version of this page transparently to the end user without the need for an application rewrite.
Answer: A proxy is (essentially) an intermediate for a transaction or request.  Standard network usage of "proxy" is for an intermediate which protects the identity/location/etc. of the request creator.  A "reverse proxy" is an intermediate which protects the request target. A "transparent proxy" does not protect either side.
Other technology (such as load balancing, packet filtering, caching, etc.) can be combined with proxy technology to (as noted by others) dramatically improve security and performance.
Answer: A reverse proxy is so called because it acts as a proxy for incoming requests from outside the LAN.  A normal proxy (e.g. Squid or MS ISA) acts as a proxy for outgoing requests from within the LAN.
Normally you would use a reverse proxy for load balancing or security.  In the former case it accepts incoming requests (this is a relatively lightweight process) and forwards the requests to one or more web servers which do the actual work.  In the latter case it can be used to filter requests from unauthorised sources.

What is a Reverse Proxy?


How do I show there are no elementary function solutions for the differential equation $f''(x)=f(\sqrt{x}), x>0$?

Question: How do I show there are no elementary function solutions for the differential equation $f''(x)=f(\sqrt{x}), x>0$ in the $C^2(0,\infty)$ space solutions?
Comment: $f: x \mapsto 0$ is an elementary solution in $C^2$.
Answer: Assume that a solution $f$ lies in $C^2(0,\infty)$: then $f''\in C^2(0,\infty)$, so $f\in C^4(0,\infty)$ and so on, so $f\in C^\infty(0,\infty)$.
By using the power series method, it is easy to prove that there are no non-constant analytic solutions in a right neighbourhood of zero, so all the $C^2$ solutions belong to $C^\infty\setminus C^\omega$, and there are no "elementary" functions that belong to this strange space.
Obviously, this strongly depends on the meaning of "elementary".
Answer: Expanding on Jack D'Aurizio's answer,
write the equation as
$f(x^2) = f''(x)$.
If $f(x) = \sum_{n=0}^{\infty} a_n x^n$,
$f(x^2) = \sum_{n=0}^{\infty} a_n x^{2n}$
and $f''(x) = \sum_{n=2}^{\infty} n (n-1)a_n x^{n-2}
= \sum_{n=0}^{\infty} (n+2) (n+1)a_{n+2} x^n
$
so $a_{2n}  = (n+2) (n+1)a_{n+2}$
and $a_{2n+1} = 0$.
Setting $n = 0$,
$a_0 = 2a_2$.
Setting $n = 1$,
$a_2 = 6a+3 = 0$,
so $a+0 = 0$.
Setting $n = 2$,
$a_4 = 12 a_4$, so $a_4 = 0$.
If $n = 2k+1$, $a_{4k+2} = 0$.
In particular $a_6 = 0$.
For $n = 4$, $a_8 = 30a_6 = 0$.
Suppose there is a $n > 4$ for which $a_{2n} \ne 0$.
Let $m$ be the smallest such $n$.
Then, since $2m > m+2 > 6$,
$a_{2m} = (m+2)(m+1)a_{m+2} = 0$.
Therefore there is no such $m$,
and $a_n = 0$ for ann $n$.
Thus the only solution is $f(x) = 0$
if $f(x)$ has a power series expansion.
If $f(x) = a x^b$,
$f''(x) = a b (b-1) x^{b-2}$
and $f(\sqrt{x}) = a x^{b/2}$.
For this to be a solution,
$a = a b(b-1)$ and $b/2 = b-2$
or $b=4$ and $a=0$,
so there is no solution of this form.
There may be a non-zero solution not of these forms,
but I do not know of any.
Comment: In the beginning, did you replace $x$ with $x^2$?
Comment: The question is about solutions on $(0,\infty)$.  There seem to be nontrivial solutions there.
Answer: Integrate from $1$ to $x$: the functional equation becomes
$$ \int_1^x f''(t)\ dt = \int_1^x f(\sqrt{t})\ dt $$
$$ f'(x)  = f'(1) + \int_1^{\sqrt{x}} 2 u f(u)\ du $$
Integrate again:
$$ f(x) = f(1) + f'(1) x + \int_1^x \int_1^{\sqrt{t}} 2 u f(u)\ du \ dt
= f(1) + f'(1) x  + \int_1^{\sqrt{x}} 2 (x - u^2) u f(u)\ du$$
and I think we should be able to use the contraction mapping theorem to get solutions of this in an appropriate space of functions.
EDIT: There are two linearly independent solutions in power series around $x=1$:
$$ 1+\frac{1}{2} \left( x-1 \right) ^{2}+{\frac {1}{96}}\, \left( x-1 \right) ^
{4}-{\frac {1}{320}}\, \left( x-1 \right) ^{5}+{\frac {61}{46080}}\,
 \left( x-1 \right) ^{6}-{\frac {863}{1290240}}\, \left( x-1 \right) ^
{7}+{\frac {62581}{165150720}}\, \left( x-1 \right) ^{8}-{\frac {
2767907}{11890851840}}\, \left( x-1 \right) ^{9}+{\frac {192672359}{
1268357529600}}\, \left( x-1 \right) ^{10} + \ldots
$$
and
$$ x-1+\frac{1}{12}\, \left( x-1 \right) ^{3}-{\frac {1}{96}}\, \left( x-1
 \right) ^{4}+{\frac {7}{1920}}\, \left( x-1 \right) ^{5}-{\frac {73}{
46080}}\, \left( x-1 \right) ^{6}+{\frac {2087}{2580480}}\, \left( x-1
 \right) ^{7}-{\frac {76093}{165150720}}\, \left( x-1 \right) ^{8}+{
\frac {6754511}{23781703680}}\, \left( x-1 \right) ^{9}-{\frac {
706941917}{3805072588800}}\, \left( x-1 \right) ^{10} + \ldots
$$
I would expect these series to have radius of convergence $1$, with singularities at $0$, but the solutions should extend analytically to $(0,\infty)$.
EDIT: if $f$ satisfies the equation on $(a,b)$, $0 < a < 1 < b < \infty$, then
 $f(1) + f'(1) x  + \int_1^{\sqrt{x}} 2 (x - u^2) u f(u)\ du$ satisfies the equation on $(a^2, b^2)$, and agrees with $f$ on $(a,b)$.  So if there is a solution on a neighbourhood of $1$, there is a solution on $(0,\infty)$.