Is it safe to drink urine when you have nothing else to drink?

Question: What if there was a situation where someone had no water to drink or food to eat. Would it be safe for them to drink their own urine? What are the possible risks of doing this? Could there be any benefits?
Answer: If you are in a no-other-liquid situation then not, it's not safe to drink. The problem is that your urine is a way to remove superfluous minerals and other stuff from the body. The body's job, especially in a state of dehydration, is to remove this waste with the least amount of water sufficient to flush it away.
If you then drink this urine, you are ingesting back all the salts you just got rid of. You are not getting your liquid-to-salts ratio better, you are making it worse. So when you are already dehydrated, you get dehydrated even quicker if you drink your urine.
If you are at no risk of dehydration (you can additionally ingest sufficient water from drinks or food), then the above does not apply. As long as you don't have an urinary tract infection, your urine is also sterile, so you are not at risk of infection either. This makes urine safe to drink.
There are people who drink urine (outside of dehydration situations) for supposed health benefits, but I have never heard of any evidence that such benefits exist.
I don't know of peer reviewed sources for this, but the dangers of drinking urine for survival are both logical from a theoretical view point and common knowledge in outdoor lore, for example see http://survivalcache.com/water-purification/.
The Outdoors site has the same question, with a suggestion for a possible purification method which might remove the dissolved minerals. But urine is also rich in organic compounds and starts breeding bacteria almost immediately upon leaving your body. So I'd be weary to try it: fresh urine is safe, but urine which has stood around at room temperature isn't.
Comment: Urine is also a way of removing excess water. I say this because one urinates more when they drink more water.
Comment: Based on survival training I've had (albeit many years ago), I don't believe your answer is entirely correct. The avalanche survival site you linked to agrees with what I was taught; ie, that drinking the first two "passes" is safe and will extend your survival time, but after that it will be detrimental for the reasons you cite. It's also unlikely you will have more than two opportunities anyway. If you're in a situation where drinking urine has become your only option, it's unlikely you'll produce more than one or two full bladders anyway.


Ford KA won't start

Question: I tried to start my 2007 Ford KA this morning but it makes a very high pitched screeching sound. Almost like a grinding.
The car won't start at all.
Could you please advise on what this could be?
Thank you
Comment: Was the vehicle starting without any issues prior to this event?
Answer: This is usually the telltale sign of a weak battery. The grinding sound is typical when the starter motor isn't provided enough power to turn the engine over.
Have the battery tested to confirm that it is weak prior to replacing it.
Answer: Sounds like it could be a worn pinion on the starter motor or worn teeth on the ring gear on the fly wheel.  If you feel confident, you could - turn the ignition off, put the car in 4th gear (this will only work for a manual car), take your foot off the clutch and take the handbrake off, then try to nudge the car a little to get a different set of teeth on the flywheel exposed to the starter.  Put the handbrake back on, take it out of gear and try to restart.  If it now starts, then you need to get the starter/flywheel ring gear checked/fixed.
Best try this with someone else to give the car a nudge while you are in the car ready to press the foot brake if needed.  Don't try this if the car could roll away on a slope.  And only try it if you fully understand what you are trying to do.
It could also be just a low battery as Zaid suggests.
Answer: Well when you start most cars, you close a circuit that magnetically pushes the starter gear to come in contact with the engine and rotate it. After the engine successfully starts the starter moves back to it's position. Since you describe the sound to be similar to "grinding", this could either mean that the starter gear is worn out (the starter moves properly but the gear is worn out and can't rotate the engine) or the starter is stuck and won't move into position at all, or touches the engine just enough to produce the "grinding sound". The latter (which is more likely) could be due to weak magnetic field (maybe cause of a weak battery, or an electrical fault) or due to a mechanical failure that prevents the proper movement of the starters head.
You can't really do many things without taking out and examining your starter but try these.
First try to start the car, by rapidly and firmly turning the key. pause for at least 10 seconds in between. Don't insist if it won't work instantly and don't try it more than 2-3 times.
Add a battery booster or assist the starter with a little extra charge from your friend's car. Again don't insist more than 2-3 times.

Ford KA won't start


Where are custom kernels being flashed to in internal memory?

Question: I've unzipped zip files for custom kernels and noticed that the majority of the files that are being applied are in a /kernel folder. However that folder is unlisted when listing the root directories using both Root Explorer and ES File Explorer. Where is this /kernel folder?
Answer: The kernel is part of the "boot image", which is stored in a separate partition in the flash memory, not in a normal file. See http://android-dls.com/wiki/index.php?title=HOWTO:_Unpack,_Edit,_and_Re-Pack_Boot_Images
Answer: May be you can explain a little bit more on what you are trying to do..
Also, if you can list those custom kernel files, we can help better.
If I need to flash a custom kernel, I follow below process:

Get the source from the git repository
Make my changes to the kernel source. Change could vary from adding a simple debug printk statement to adding a new driver.
Build the kernel source using cross-compiler toolchain
Change the phone to Fastboot mode (I'm talking bout G1 phone)
Use 'fastboot' to flash the custom kernel (zImage) using command: 'fastboot flash:raw boot zImage ramdisk.img'
Use 'fastboot reboot' command to reboot the phone using the new kernel

Hope there is some clarity now.


What is Typical RTP Packet size for H.264 video transmission

Question: I am encoding my video using JM reference software. The encoded file is divided into RTP packets. I can set a RTP packet size in bytes using configuration file. But i am unable to find what should be the RTP packet size for H.264 video used in video telephony? I know RTP packet size is variable but there should be some limit.
Any help would be highly appreciated.
Answer: RTP Payload Format for H.264 Video
<code>   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |V=2|P|X|  CC   |M|     PT      |       sequence number         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           timestamp                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           synchronization source (SSRC) identifier            |
  +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
  |            contributing source (CSRC) identifiers             |
  |                             ....                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    RTP header according to RFC 3550
</code>
The RTP header information to be set according to this RTP payload
   format is set as follows:
Marker bit (M): 1 bit
      Set for the very last packet of the access unit indicated by the
      RTP timestamp, in line with the normal use of the M bit in video
      formats, to allow an efficient playout buffer handling.  For
      aggregation packets (STAP and MTAP), the marker bit in the RTP
      header MUST be set to the value that the marker bit of the last
      NAL unit of the aggregation packet would have been if it were
      transported in its own RTP packet.  Decoders MAY use this bit as
      an early indication of the last packet of an access unit, but MUST
      NOT rely on this property.
<code>     Note: Only one M bit is associated with an
     aggregation packet carrying multiple NAL units.  Thus, if a
     gateway has re-packetized an aggregation packet into several
     packets, it cannot reliably set the M bit of those packets.
</code>
Payload type (PT): 7 bits
      The assignment of an RTP payload type for this new packet format
      is outside the scope of this document and will not be specified
      here.  The assignment of a payload type has to be performed either
      through the profile used or in a dynamic way.
Sequence number (SN): 16 bits
      Set and used in accordance with RFC 3550.  For the single NALU and
      non-interleaved packetization mode, the sequence number is used to
      determine decoding order for the NALU.
Timestamp: 32 bits
      The RTP timestamp is set to the sampling timestamp of the content.
      A 90 kHz clock rate MUST be used.

H.264 also implements B-Frames. This type of frame fills in information between P-Frames. This means that the B-Frame will need to be held until the next P-frame arrives before the information can be used. B-Frames are not used in all modes of H.264. The encoder decides what type of frame is best suited. There are typically more P-frames than I-frames. Lab analysis has shown TelePresence I-frames to generally be 64K wide (50 packets @ 1316 bytes), while P- frames average 8K wide (9 packets at 900 bytes).
Hope this helps
Comment: very much thansk for your answer. I already have an idea about RTP header szie and format. But the thing i want to know is written in your last sentence. could you please elaborate it more? like your'e saying for P frame its 9 packets at 900 bytes . Does it mean 1 packet of 900 bytes? I want to know what is normal RTP packet size in total  which is used by video calling applications etc.
Comment: Any frame can be divided into multiple packets, so it would be smaller. You need to check the MTU (maximum transmission unit) of your network.


configuring dns to point to my hosted machine

Question: I want to check if I've configured things correctly. I've registered a domain at godaddy.com and created a virtual linux server at hostvirtual.com. In the godaddy dns manager interface I've added the nameservers ns1.hostvirtual.com and ns2.hostvirtual.com. At hostvirtual.com they have a dns zone editor where I've added an A record that maps the hostname of the virtual machine to its ip address. Is this right? Or do I have to add the mapping in the zone editor at godaddy.com?
Answer: As I believe it now stands, you have two sets, so you need to update both places.
If you just added the NS from hostvirtual without removing the GoDaddy ones you will need to change GoDaddy as well.  If you removed the GoDaddy name servers, the entries there would be irrelevant.  Alternatively, you could have left just the name servers at GoDaddy and added an A record through their interface.
Answer: You point your virtualhosting name server from godady.That means you should use virtualhosting name servers' interface to create your dns records. Some domain registrar are also provides name server (such as name cheap) and you don't need to point any external nameserver but just use their servers to create dns records.
Shortly what you did was right, you need to keep your dns records at your hosting company's site.
Answer: Everything looks fine. Can you elaborate exactly what problem you are facing in above setup so that we can respond accordingly.


Which apps show up in the market on an Android tablet?

Question: Is the market filtering the same as a phone (auto detects required features and OS version) or do developers need to specifically set attributes on their apps for them to be available on tablets running Android?
Answer: From what I read a few days ago, tablet cannot run Android Market (and therefore will not be distributed with it) until 2.3 is released in the early part of next year. I have heard of a few tablets being released using the manufacturers own market-type application but nothing using the official Android Market.
I think we'll have to wait until 2.3 is released before we hear about tablet support =(
Gary
Comment: thanks.  makes sense why they're pushing out all the new market changes including allowing devs to upload hi-res graphics and link to youtube videos.  that's probably mostly for tablets
Answer: Since the answer from gsteinert is a little bit outdated, I will try to give an update:
tl;dr
Apps on tablet will go to the same filter process as apps on phones on the Market.
Apps that appear on tablets follow the same filtering rules as with phones. Developers are given the ability to set attributes for their app in the, so called, manifest. For example: If an app declares that it's, for whatsoever reason, only compatible with small screen sizes, it will not show up on the Market on a tablet with greater screen sizes. Note that this case is very rare, because most apps that are only designed for small screens will either be upscaled or just use a small amount of the screen.
There is also a way to declare an app for honeycomb tablets only.
Answer: Again an update, as meanwhile Flow's answer also got a little outdated (LOL):
If you want to filter for good tablet optimized apps, for one there are specialized apps like Tablified Market or Tablet Market to help you out.
On the other hand, Google started to work on this issue meanwhile. So you find a tab titled Featured Apps for Tablets at least on the Google Play website. For details on this, you might also wish to read Google Play tablet apps getting special designation; according to that article, the Google Play Store is now only showing tablet-optimized apps in its top lists when accessed through a tablet.


How do you refer to a note that is more than one octave above or below middle C?

Question: I know you say the A above Middle C when refering to the A that is in the same Octave as Middle C in the Treble Staff, or you say the A below Middle C when referring to the A in the previous Octave on the Bass Staff, but what about if you're referring to the A in the next or previous Octave and so on? Do you say two As above Middle C or two As below Middle C or something like that?
Answer: This is an issue of what we call octave designation.
There is actually an international standard here: called International Pitch Notation (IPN), it labels Middle C as C4. An octave above Middle C is C5, an octave below Middle C is C3, etc.

In short, the C's octave range is in play until the next C changes the octave register. In other words, from C3 to B3 is all X3; it's not until C4 that pitches enter the X4 octave.
The G below Middle C, for instance, is G3, because it's within the C3 to B3 range. The G at the top of the treble clef will be G5.
Lastly, sometimes accidentals can be a little tricky. Imagine the B♯ that's enharmonic to Middle C. It's raised from B3, but it's enharmonic to C4. In this case, we focus on the note name. Since it's a chromatic alteration of B3, it's B♯3, even if it's enharmonic to C4.

I should also mention another system, that of Helmholtz notation, which uses a mixture of capital/lowercase letters and sub/superscript slashes:

It functions in a very similar way that the above IPN does, it's just a different labeling system.
In my experience, C4 as Middle C is very much the standard.
Comment: The notation I know uses underscored C for C contra (then double underscored for subcontra).
Answer: In addition to Richard's answer you might also consider MIDI note designations which work similarly. In MIDI there are 128 notes split into 10 octaves that start with C0 (#0) and going to G10 (#127) with every octave designation starting on C and moving up every 12 notes. This would put middle C at C5 (#60) rather than C4, and in IPN that makes octave 0 into octave -1. Of course, a normal 88 key piano can only play from note #21 (A1) to note #108 (C9), but if you're looking to compose using the computer can can be helpful to know these designations.
Comment: Referring to middle C as MIDI note #60 would seem reasonable, but I don't think MIDI octave numbers would be used except perhaps in some MIDI transcription programs.  Even there, I think note #12 is the lowest valid C note in MIDI.
Comment: @supercat could you clarify more on "*note #12 is the lowest valid C note is MIDI*"? Looking at the table listed [here](https://www.midikits.net/midi_analyser/midi_note_numbers_for_octaves.htm), there's #0 for C-1. And as explained further, the lowest octave number can vary from -2 to 0. But still, MIDI note #0 is valid.
Comment: @AndrewT.: It's been decades since I got my first MIDI keyboard and software, but I thought I recalled something using a note value of 0 as having a special meaning.  Maybe it's only a velocity of zero that's special.  Mea culpa if so.  In any case, i suspect 60 was chosen as middle C because it's closest to the middle of the 0-127 range, rather than because there were actually 5 usable octaves above and below it.  If C5 were 256Hz (a bit below concert pitch), C0 would be 8Hz and C10 would be 8192Hz.  Rather extreme frequencies.
Comment: @supercat ah, I think I understood the underlying issue. Actually, the MIDI note itself doesn't mean anything in term of frequency. The underlying instrument/VST that plays the MIDI note is the one that controls how to play each note. An example with piano would be not playing any sound for notes C9. However, MIDI is also used for making [drum sounds](https://en.wikipedia.org/wiki/General_MIDI#Percussion), or even [sound effects](https://en.wikipedia.org/wiki/General_MIDI#Sound_effects). C0 on those "instruments" won't play "8Hz" ;)
Comment: @supercat That's kind of the point of MIDI. I think more importantly MIDI data is encoded in 8-bit so having 128 notes gives you half the values that 8 bits provides. Sub frequencies can be utilized by some speaker systems, most likely for bass drums, but as Andrew says the pitches aren't bound by frequency so you could use a different tuning too.
Comment: The point of MIDI is to be able to control synthesizers that were built with early-80s' affordable components. Hence the puny 7-bit numbers and all the other ridiculous limitations. It is a sad testament to the innovation scope in music technology that this standard is still used so widely; really it should have been disbanded decades ago in favour of something better.
Answer: Sometimes there is benefit in losing some precision for the sake of simplicity. There are lots of good answers here but in rehearsal and when talking to students I find myself using a simpler way of communicating which octave I mean. I believe I learnt this from my first piano teacher.
The C in between the treble and bass clefs is middle C; the C in the middle of the treble clef is Treble C; the C in the middle of the bass clef is Bass C. There C on ledger lines above the treble clef is High C, the one below the bass clef is Low C.  This gets 5 octaves which works well for voice and most instruments, especially in early levels.
To refer to non-C notes, I use above and below referring to the nearest C, so "the F above Low C" or "the G below Treble C."
I find this the easiest for spoken communication especially with students I work with a lot. For writing I generally default to IPN in Richard's answer.
Answer: If you'd like to name a note relative to middle C, it would probably be "2nd A above/below middle C". Many articles on English Wikipedia provide this kind of naming together with International Pitch Notation one.
Middle C is C4 in IPN so its octave can be called "fourth". However some MIDI-related software shift octave numbers up by one (probably because MIDI note 0 is C-1 and they want to display it as "C0" instead), for example middle C is "C5" in FL Studio. Some electronic keyboards have the opposite shift and middle C is labeled "C3" on them. This variation can lead to confusion.
Helmholtz notation provides different octave names. From middle C and up octaves are one-line (or once-accented), two-line (or twice-accented) and so on. Octaves below (from higher to lower) are called small, great, contra, sub-contra. No number variations, but naming of lower octaves isn't very intuitive.


How do you select an edge loop with BMesh?

Question: Can someone please explain to me how I go about selecting edge loops in BMesh? I'm new to Blender scripting and am venturing into BMesh but I can not figure out how to actual select an edge loop. Let's say I'm starting with an edge. How then do I go about getting and selecting the edge loop that that edge is a part of? Please and thank you!
Answer: This is almost a duplicate of this question. Simply add the select statement at the end. I also branched the code sample to go in both directions of the first edge.
<code>e = bm.edges[0]

for loop in e.link_loops:
    if len(loop.vert.link_edges) == 4:
        e.select = True

        while len(loop.vert.link_edges) == 4:
            # jump between BMLoops to the next BMLoop we need
            loop = loop.link_loop_prev.link_loop_radial_prev.link_loop_prev

            # following edge in the edge loop
            e_next = loop.edge
            e_next.select = True
</code>
Comment: Awesome, thanks! I saw that one but for some reason I couldn't ever get it to work let alone how it actually works.


Is there a way to continue a "Survival: Endless" game?

Question: Is there a way to continue a "Survival: Endless" game?  I am up to flag 27, but carelessly lose the level... Do I have to start all over from flag 1 to flag 27 again?  That's very tiring!
(Update: actually, I thought of a way on PC or Mac, which is to install the game on a virtual PC, so we have a "snapshot" of the hard drive.  We can just quit the game, and save any number of copies of the virtual hard drive, and be able to start the game as on that drive's image.... but probably we can save some data file instead of the whole hard drive).
At flag 27, I forgot to include a Coffee Bean to wake up Ice-Shroom and Doom-Shroom and the Gargantuar and Giga-Gargantuar will end the level right there -- even invoking the Lawn Mowers first, and then a second wave wil end the game.  I usually pack the Lily Pad, Doom-Shroom, and Coffee Bean first, and then Ice-Shroom and an Imitator Ice-Shroom, but sometimes I have to do repair work and then forgot to pack something.
I am playing the iPad version (version 1.9.4 as of June 2012).  The set up I am using is the cobless one:
(by the way, what is the "white flower" on the ground at the right side of the lawn?)
Answer: No matter which wave you lose at, you have to restart from the beginning. Also, the white flowers are daisies and have no purpose but to be an easter egg (type daisies on your keyboard -- I don't know how to achieve it on iPad).


Is steganography valid only for digital images or also for printed images?

Question: If I embed some information in an image using steganography and then I print that image, will I be able to retrieve information from the printed image or not?
Comment: No luck with my forensics paper in https://crypto.stackexchange.com/a/70233/23115 ?
Comment: Don't you remember from the history the hair of the messenger is cut, the message is written the to the bald head, then waited for the hair grow, and after that, the messenger allowed to travel and in the destination again cut to release the message.
Answer: It depends on the steganography method you're using, and on how the image is printed.
Most steganography methods designed for digital images probably won't survive printing and scanning the image, or at least doing so will introduce a significant amount of noise into the concealed data that will exercise the error correction pretty hard and significantly limit the maximum payload capacity.
However, there are also steganography and watermarking methods that are designed to survive printing, or even designed specifically for printed images.  Indeed, you may have already unknowingly included such watermarks in your prints, if you've printed them on a printer that embeds a machine identification code in its output.
Answer: Most certainly no. If you are using a "classic" LSB based algorithm with uncompressed images you would have to be able to scan the printed image and end up with the exact same pixel values, which I don't think is possible. I mean the scan would have to result in the same file. At least if you are using a "good" algorithm which does not alter the cover image in any noticeable manner.


Can you freeze mirepoix?

Question: Can I chop up and freeze mirepoix raw, or do the vegetables have to be blanched first?
Thanks!
Answer: Yes, you can freeze mirepoix, but it'll be pretty soft once defrosted, as Peter V noted. Onions and celery do not freeze well, though carrots do okay.
All three vegetables, when whole, should be blanched for a few minutes before freezing. For vegetable-specific freezing instructions see the National Center for Home Food Preservation's Freezing Guide.
It's best to use fresh, but blanched for 1-2 minutes then frozen works okay. I do not blanch diced vegetables for more than 2 minutes.
Answer: What will you be doing with this mirepoix?
If it's just going to be a basis for a stock, you could probably freeze it without ill effect.
If you're going to be caramelizing it in some butter, and then using it in a final dish, I'm not sure that I would freeze it; the veggies seem likely to get mushy, regardless of whether or not they were blanched first.
Answer: If you are planning on using the mirepoix as-is, as in it will be seen on the plate, yes you should blanch first and then freeze as fast as you possibly can. This will help retain texture.
Otherwise, if you're using for stock or whatever, go ahead and freeze.

Can you freeze mirepoix?


No Unity 3D in VirtualBox

Question: I'm having trouble running Unity 3D in Ubuntu 12.04 LTS in VirtualBox.
I tried enabling 3D acceleration, it still does not show 3D. Only 2D.
I tried installing VBox Guest additions, won't show 3D.
The Ubuntu live CD session: no 3D.
Comment: Welcome to Ask Ubuntu! What version of VirtualBox do you have?
Answer: The 3D in VBox is not enough to run Unity 3D.
Most Virtual Boxes have very poor support for 3D, at least the more advanced aspects like transparency and the like. I use VMWare Workstation and it has no issues. I never managed to get VBox's 3D working sufficiently to even run a simple game like Minecraft.

No Unity 3D in VirtualBox


Why aren't there galaxy-sized balls of iron out there?

Question: Since we know that stars burn out, collapse, or blow up when too many of their atoms fuse into iron, then how come, with all the ancient galaxies we've been observing, there isn't one galaxy whose stars have become giant balls of iron, which would eventually collide into one galaxy-sized iron globule in space?
Comment: Your hypothesis is not correct: not all stars produce iron. Conversely, the most of them have not enough mass to do it.
Comment: _meta:_ I also recommend tags like `star-death` and `star-lifecycle`
Answer: In short, you can't make a galaxy-mass ball of iron because there's no way to support the ball against the inward crush of its own gravity. First, it would collapse into a neutron star, but even neutron stars can't hold themselves up beyond about 2-3 solar masses.  Your galaxy-sized object would collapse into a black hole.  But we do see these!  All massive galaxies are believed to have supermassive black holes at their centres. These are up to 10 billion solar masses in size, which is comparable to smaller galaxies.
In any star, be it a main-sequence star like the Sun, a giant, or even a white dwarf or neutron star, the inward force of gravity must be balanced by some outward force or pressure.  (In fact, we basically assume this when we construct stellar models, although it's a very good assumption.) In main-sequence stars, this pressure is provided by the nuclear reactions happening inside them but as a star evolves, it gets a bit more complicated. In some situations, the nuclei get so close together that they their electrons begin to share quantum states. And because no two electrons can be in the same state, they have to occupy higher energy states. This in effect exerts a kind of pressure (called degeneracy pressure), because the electrons are forced to move around faster than they otherwise would. This is what supports white dwarfs and the cores of some evolved stars. e.g. low-mass red giants.
Degeneracy pressure is fine up to a limit known as the Chandrasekhar limit, which is about 1.44 solar masses (depending on the composition of the object in question). Anything above this would collapse. First, this collapse would yield a neutron star, where things are a bit different. But a similar principle holds, and there's a degeneracy pressure because the neutrons themselves also occupy one quantum state each.  The details are much more difficult here, but the overall consensus is that neutron degeneracy pressure can support objects up to about 2-3 solar masses. After that, there's nothing left to balance gravity, and the star collapses into a black hole.
Answer: Apart from the good reasons given in the other answers as to why a ball of iron in excess of about $1.4$M$_\odot$ cannot be stable, there is another reason. Namely, there is no way to form that much pure iron. Iron is produced in supernovae, but only a fraction of the matter expelled by a supernova is actually iron and there is no natural way to select is. The remaining matter is locked in the stellar remnant, either a white dwarf, neutron star, or (stellar-mass) black hole. The latter two are already to massive for the material to be in the form of iron, while the a white dwarf may only contain some iron, but never is a ball of iron.
Answer: If you had a galaxy sized ball of iron the gravitational force emitted would be immense, and without any force pushing out from it the ball would collapse into itself to form a black hole. In stars such as the sun (main-sequence stars) the outward force of nuclear reactions taking place in its core keep it from collapsing into itself. However, with a ball of iron the only force acting on it would be the pull of gravity, eventually crushing the ball into itself.
The ball of iron wouldn't even be able to reach the size of a galaxy. There is a limit known as the Chandrasekhar limit, which says anything that is about 1.44 solar masses would collapse. This would most likely form a neutron star, but this also has a limit to the amount of mass it can hold. Eventually, even this will collapse upon itself to form a black hole.
Comment: Isn't this essentially what [@Warrick said about an hour ago](http://astronomy.stackexchange.com/a/6167/373)?
Comment: It is similar to his answer. We both had the same ideas
Comment: The "Chandrasekhar mass" for a ball of iron is more like 1.2 solar masses.
Answer: Iron spheres
There is a hypothesis that after about $10^{1500}$ years (many star generations) iron stars (iron sphere star remnants) will form. This would require all the elements lighter than Fe to fuse into Fe and all the elements heavier than Fe to decay into Fe. Also proton is required not to decay for this hypothesis to work.
Mass limit
Only stars under certain mass limit can possibly form a sphere of iron. As others pointed out after the nuclear fusion ceases stars heavier than 1.44 solar masses will collapse into a neutron star and above several solar masses they will further collapse into a black hole.
This means that iron spheres considerably heavier than the solar mass cannot form because the iron nuclei of heavier objects will be crushed to mostly neutrons or some other state of matter not consisting of iron.
References for the iron star hypothesis

iron star on Wikipedia
Dyson, Freeman J. (1979). "Time without end: Physics and biology in an open universe". Reviews of Modern Physics 51 (3): 447–460. Bibcode:1979RvMP...51..447D. doi:10.1103/RevModPhys.51.447.
Dyson, Freeman J. (1996). "Selected Papers of Freeman Dyson with Commentary", American Mathematical Society
Comment: The maximum mass of a ball of fully ionised iron, supported by ideal electron degeneracy pressure, is around $1.2 M_{\odot}$. The limit of 1.44$M_{\odot}$ you quote is too high even for a carbon/oxygen white dwarf - it is more like 1.38$M_{\odot}$. The outcome of a C/O WD exceeding this is almost certainly a type Ia supernova, not a neutron star. However, I concur that an iron WD that exceeds its Chandrasekhar mass will likely end up as a neutron star.
Comment: The last paragraph is really only true for stars that cannot undergo nuclear fusion any longer. Thanks for citing sources! I've been hoping for an uptick in citations.
Comment: @HDE226868 thanks for the note. I thought that it was obvious (also from the first paragraph) that the collapse happens when the fusion activity decreases. I have edited the answer to explicitly mention the end of the fusion. --- The second paragraph is only to mention the star mass limit for the Fe nuclei not to be destroyed by the star collapse.
Answer: Our sun will eventually become a white dwarf. A star 10 times it mass will become a neutron star. A star 100 times the mass of the sun will become a black hole. So if 100 stars came together into one mass, it would collapse into a black hole. It is currently believed that a super massive black hole (at least 1000 times the mass of our sun) is at the center of our galaxy and also in the Andromeda galaxy. This is where all the iron might be found from this region of space.
Comment: You wouldn't need 100 stars to come together to make a black hole. Also, the masses of the two black holes mentioned are way more than 1,000 solar masses. And would an atom of iron really survive deep inside a black hole?
Comment: @HDE226868 I knew that. I just didn't want to take the time to look up the correct amounts.


When do binomial distributions occur?

Question: A binomial distribution is the distribution of the number of successes of n independent, identical Bernoulli trials.  What happens when the trials are dependent and the Bernoulli trials are not identical by which I mean that the probability of success from trial to trial varies?  How identical and close to independent do they have to be before we see something that resembles a binomial distribution?
Answer: You are asking, I think, when a Central Limit Theorem holds.  The simplest form of the CLT is that the binomial distributions Binomial(n,p), suitably rescaled, converge to a normal distribution as n goes to infinity.  (This binomial case is usually not called the CLT, but goes under the name of the de Moivre-Laplace theorem.)
Now, a Binomial(n,p) random variable is the sum of n Bernoulli(p) random variables.  The usual form of the CLT states that if $S\_n = X\_1 + ... + X\_n$, where the $X\_i$ are independent and identically distributed with mean μ and standard deviation σ, then $(S\_n - \mu n)/(\sigma \sqrt{n})$ converges in distribution to the standard normal as n → ∞.
If the $X\_i$ are in fact dependent, see the link provided by Ori Gurel-Gurevich above.
If the $X\_i$ are independent but not identically distributed, then there are two standard conditions for proving that the rescaled distribution of $S\_n = X\_1 + ... + X\_n$ converges to the standard normal: Lindeberg's condition and Lyapunov's condition.  Both are a bit difficult to understand when you first look at them.  But the basic idea behind both of them is that if no one of the summands $X\_i$ is too large (in variance) compared to the others, then the normal distribution still appears.
Answer: Depending on your setting, it might better to consider the asymptotic when $n\rightarrow \infty$, which gives you a Poisson distribution. There are various conditions, weaker than independence, under which Poisson approximation holds. For example, take a look here.
Comment: Also at Pemantle's web page (http://www.math.upenn.edu/~pemantle/full-year-webpage.html) are links to two articles by Arratia, Goldstein, and Gordon on which the lecture you linked to is based.
Answer: The distribution you describe is called a Poisson-binomial distribution.  If you do a search with this name you will find a substantial literature on it.

When do binomial distributions occur?


FTP transfer hangs for random files

Question: I've been stuck on this FTP issue for a while now. I have IIS 7 setup with an IIS 6 FTP server running on a Windows Server 2008 box. The problem I am running into is that I can't download certain files from the FTP server, even though I uploaded those files to the FTP server. The connection times out after 120 seconds. I have used Wireshark and checked the log files. The only message I see is the timeout message. The first thing that came to my mind was permission issues, however I have probably tried every combination of permissions that I can think of, with the end goal of getting the permissions to be the same for the files that work and the files that do not work. With the list of files I have now, I can download the zip, war, and msi files, but not the txt or sql files. It almost seems like a binary thing, but I've changed my transfer mode on the FTP client and also toggled the Active/Passive options around.
Answer: Another troubleshooting step would be to temporarily disable real-time AV on the server and/or the client.  Have you tried different FTP clients?
Comment: I have tried the Windows command line client and FileZilla.
Answer: Check if those files are not in use. You can use Process Explorer for that (Find handle or dll...).

FTP transfer hangs for random files


Differences between Quorum and Pantheon

Question: I want to build an app on a private blockchain and I'm hesitating between 2 clients : Quorum and Pantheon (now Hyperledger Besu)
Does anyone know what are the main differences between the 2 and the use case for each one?
Answer: There are 2 main differences:

Besu is a more generic client that support Ethereum mainnet and is written in Java
Quorum is a fork of geth (go-ethereum, the most used client for ethereum). It does not support mainnet as of now, and as a fork of geth it is written in Go

There are many other differences in terms of ease of setup, ease of use, and various small differences in supported features such as supported consensus algorithms, and enterprise features.
Edit: theres another difference I forgot to mention. While both projects are open-source, Besu is Apache license like all Hyperledger projects, while Quorum inherits LGPL license from geth
Comment: Thank you for your response! Do you have anything in mind when you talk about entreprise features?
Comment: Yeah, theres a ton of difference in those but I think the main thing to note here is that Quorum tooling is open source while Pegasys is planning to separate anything they consider enterprisey and sell it as a package separate from the OS project. Note: this is my understanding as of now.

Differences between Quorum and Pantheon


Is there a difference in the Red Hulk's and Green Hulk's powers?

Question: I get that the two Hulks are different characters, but is there a difference in the Red Hulk's and Green Hulk's powers?
Answer: Yes, there are important differences between the two characters despite their similarities.
At the basic level, both the Red Hulk (former General Thaddeus "Thunderbolt" Ross) and the Green Hulk (Dr. Bruce Banner) are very similar. They both have vast superhuman strength and are arguably two of the strongest mortals on Earth. They have an incredible rate of regeneration, and superhuman resistance to injury. Both have shown near-complete invulnerability to weapons of human origin. Both have shown the ability to fight metahuman opponents of vast power and have fought and defeated some of the most powerful beings in the Marvel Universe.
Where they differ is how they increase their strength past its already amazing limits:

For the Green Hulk, the angrier he gets the stronger he gets. His rage increases his physical output, his damage, his resistance and regeneration. There has appeared to be no limit to how angry the Hulk can become, so his powers appear to have no upper limit.
For the Red Hulk, his powers increase as he is exposed to radiation, and in particular, gamma radiation. He has been seen to channel and harness a variety of radiations including cosmic radiation; the greater his exposure, the more physically powerful he becomes. In battles against the Green Hulk, the Red Hulk's power would increase since the Green Hulk emits low-level gamma rays all the time.

Where they differ is the amount of radiation output both generate. While the Green Hulk always generates a certain amount of gamma radiation, the Red Hulk's energy output is significantly higher. It was discovered by the Green Hulk the more the Red Hulk fought, the hotter he became. When he reached a certain heat threshold he became disoriented and vulnerable.

From the Marvel.com wiki: Hulk uncovered Red Hulk's condition of the angrier he gets, the hotter
he gets, and guessed it could be a weakness. Hulk let his opponent
strike him repeatedly until the heat became too intense for the Red
Hulk. He became unbalanced, and Hulk was able to finish him off
quickly.

One of the other major differences is the temperament and combat skill of the two characters. Where the Green Hulk was a scientist trapped in the body of a monster, the Red Hulk was a soldier with the powers of the Hulk. The Red Hulk's sheer aggression, dominant personality and combat experience made him a far more dangerous opponent than the Green Hulk has appeared in his history.
Comment: wow great answer, I didn't know that Hulk generates radiation
Answer: In addition to what Thaddeus stated the biggest difference is Red Hulk can absorb cosmic energy which means he would pose a serious threat to Thanos and other high level cosmic beings :
http://marvel.wikia.com/Red_Hulk_%28Thaddeus_Ross%29

Energy Absorption: Red Hulk is capable of absorbing energy[16]. He
said in his confrontation against the Hulk and his Defenders that he
absorbs energy, more gamma, but that he has a real taste for cosmic
power. He has also absorbed (and taken for himself) the Silver
Surfer's power cosmic, leaving him powerless on the ground while Red
Hulk flew away on his board.[17] However, overuse of this power left
Red Hulk unable to revert to his human form.
16 According to Tom Breevort, Rulk beat the Odin-powered Thor by
drawing on Thor's own power: "Finally, we've seen it established that
the Red Hulk is fueled by cosmic energy, and what is the Odinpower if
not a form of cosmic energy? So during their battle, it's likely that
the Red Hulk was drawing on Thor's own power to fuel his fighting
strength. Brevoort's QA
17 Hulk Vol 2 #12


How To Clone part of a RPM repository?

Question: I have mechanisms for cloning RPM repos from the 'net.  But what I really want is a way to clone PART of a repo.  So far I haven't found any such existing mechanism.
Here's the requirements:

We have a local cache of all official RHEL RPMs and updates from official upstream sources.  We need to occasionally add target RPMs from a trusted third party on the 'net. Given a target package at a given repo, copy the latest and greatest version of the target RPM to our cache.
Additionally, determine all dependencies for that RPM and (here's the tricky part) if the dependencies DO NOT already exist in our local repo cache pull the missing dependencies.

The reason for the second requirement is that we only want the absolute minimal infection from outside sources in our local cache of RPMs. We want to run as much of the official RHEL RPMs as we can and only augment as needed.  The problem with blindly cloning an entire third party RPM repo is that sometimes they re-write official packages or add newer versions of official packages.  And unless those versions are direct dependencies for our target package, we do not want to those packages in our local repo cache.
The real issue is that none of the existing clone tools (mrepo is the primary example) is they blindly copy the entirety of a repo.  We don't want that for cherry picking.  If you then try to use yumdownloader or such, you find that it DOES look for dependencies but it only compares against INSTALLED PACKAGES on the current server.  You can't tell yumdownloader to compare against a local package repo.
Are there any suggestions on existing tools that can help me?
Answer: Cobbler allows partial repo mirroring, check rpm-list option, section 'REPOSITORIES' in its man page. But I'm not sure about its ability to retrieve dependencies.

How To Clone part of a RPM repository?


Can hyperfocal distance be used to keep everyone in focus in group photography?

Question: If you have different sets of people standing at different linear distances from the camera can we use the theory of hyperfocal distance to ensure sharp focus on all subjects? Conversely does being "acceptably sharp" mean that some people in front of and behind where we focus are not as sharp  portraits should be?
Answer: tl;dr - in theory, sure. In practice, I've found that it isn't really needed.
In theory, yes, one could use the hyperfocal distance (HFD from here on out) to ensure that all subjects are reasonably sharp. That being said, an important consideration for "sharpness" and HFD is the print size and viewing distance of that print. If you figured out some great range assuming a 24"x30", and then the client asks you for a 40"x60" to put above their fireplace where people will walk right up to it...well, that might not work out so well.
The use case for HFD is to get things on out to infinity into focus. Landscape shooters love it because it allows for some foreground subject to be sharp while maintaining sharpness to those distant mountains/vistas/clouds/whatever.
However, with portraits, we're typically trying to isolate the subject(s) and having some background blur isn't a bad thing. Even in environmental portraits - it's typically the immediate environment that you're showing off, not what's hundreds of yards (or more) away.
Consider this image (click to expand) (Shot by Redfield Photography):

There's obviously enough Depth of Field due to the aperture, focal length, and distance to subjects to get them all in focus - but not enough to get the background. So, this is not using the HFD. And, this would be a worse off picture had the shooter decided to use the HFD, because the background trees would serve no purpose being in focus.
Even with groups of people, like a high school marching band...something like that would necessitate using a wider angle lens from a distance, where DoF comes easy; the need for HFD is simply not there.
Comment: @L.ScottJohnson I bow down to the excellence that is your Googling.
Comment: Reverse lookup shows that Redfield Photography claims it: https://photography-on-the.net/forum/showthread.php?t=206525
Answer: In theory, only the distance focused upon yield a tack sharp image, all other distances reproduce with reduced sharpness. In practice, this is an invalid statement as their exists a span both ahead and behind the point focus that yields images acceptably sharp.  This zone (span) in photo gobbledygook is depth-of-field. Let me add, the hyperfocal distance calculations are a subset of the depth-of-field calculations.
The bases is, a camera lens works by projection an image of the outside world and that image consists of googolplex of tiny individual points of light we call circles of confusion.  So named because each has a scalloped edge and each is juxtaposed to a neighboring circle of confusion. It is the size of these circles that determines how we perceive image sharpness.
Deviating – If a friend holds up a coin you discern that coin to be disk shaped (has measurable diameter).  If your fiend backs away, as the disk of the coin shrinks. At some distance you fail to perceive a disk, it becomes a point. A point is tiny and has no measurable dimension. For the average person point of change, disk to point, occurs at 3000 diameters. In other words, a coin 1 inch in diameter appears as a point at 3000 inches = 250 feet. In metric terms, a 1 centimeter coin viewed from 30 meters.
Now the 3000 diameter distance is far too stringent for photographic applications. This is because the typical photograph has less contrast than realism and we view photographs in subdued light. Photo scientist generally agrees on 1000 diameter distance. Kodak, for critical work used 1750 and Leica 1500. Most depth-of-field tables and calculators use 1000. It's subjective anyway because no one can predict the acuity of vision of the observer nor the ambient light brightness of viewing and the contrast of the circle to its background.
That being said, consider an 8x10 image viewed from our typical reading distance. What we perceive as sharp vs. blurred will be circles of confusion smaller 3.4 minutes of arc. That's 1/50 of an inch viewed from 20 inches or 0.5mm viewed from 500mm.
In practice we view an 8x10 image made by a 35mm camera. To make the 8x10 we must enlarge the tiny camera image 8 times (magnification or enlargement = 8X). Thus the circle of confusion diameter as projected by the lens, inside the camera, must be 0.5mm ÷ 8 = 0.06mm. In other words, the images that comprise the people located on any row must be 0.06mm in diameter or smaller inside the camera.
Such circle size permits the 8X enlargement necessary to make the 8x10 image. If a 16x20 image is made and viewed from about 36 inches (1 meter) than the circle size requirement is the same. However, if a 16x20 is to be viewed from 500mm (twice as close) than the circle size in the camera changes to 0.5 ÷ 16 = 0.03mm.
Lots of gobbledygook to tell you that depth-of-field and hyperfocal calculations are laden with subjective facts not necessarily valid for all occasions. The bottom line, the point focused upon is likely tack sharp. As to distances fore and aft, the images may have reduced sharpness but our acuity of our vision is not precise, this allows such a thing as depth-of-field.
Answer:
If you have different sets of people standing at different linear distances from the camera can we use the theory of hyperfocal distance to ensure sharp focus on all subjects?

No.

Conversely does being "acceptably sharp" mean that some people in front of and behind where we focus are not as sharp portraits should be?

Yes.
Understand that hyperfocal distance is the theoretical maximum depth of field given a certain focal length, aperture setting, and "circle of confusion". Also understand that depending on your mathematical interpretation, comparative "maximum depths of field" is nonsensical. After all, focusing the lens at (or beyond) the hyperfocal distance yields a so-called "infinite" depth of field. How can one "infinity" be larger or smaller than another "infinity"? In the case of depth of field, we're not talking about Cantor sets or countable vs. uncountable infinities.
Hyperfocal distance more applies when you want both a far distant background and a near foreground to be minimally blurred. That is, you want the entire scene to be considered as a whole.
When taking a portrait, you don't care about maximum depth of field. You want maximum sharpness through the depth of interesting subjects — the people in the portrait. Without changing your aperture, you will ensure your subjects are sharper if you focus on the eyes of the person/people that are about midway the distance between the closest and furthest away in the group shot*, than if you set the camera at hyperfocal distance.

Note: The precise focus point is not exactly midway through the depth of field — for a given depth of field, the exact location of focus is somewhat closer to the lens than the halfway point. A depth-of-field calculator will demonstrate this. But for real-world shooting, if you're not breaking out the tape measures, a rough approximation is adequate.

If you take a group portrait with hyperfocal focusing, you're essentially saying that the people in the shot are only as important as the details in the far background and near foreground. But that's basically never the case: if you know or care at all about the people in the shot, they are the subject, not the foreground or background. As a photographer, it is your job to direct the viewer's attention at what you intend to show. If you are shooting people, they are the subject. Period. Focus on them. Sure, you can stop down the aperture to increase the depth of field in order to bring the foreground and/or background into relatively more focus. But don't sacrifice the focus and sharpness of the subject in order to bring nonessential things into focus. The viewer's attention is a limited resource — don't squander it by spreading the focus of the image across the entire scene. Use your tools — the sharpness of your lens — to force attention on the subject (the people), and use your art of composition, timing, lighting, etc., to add spice to the scene. But spice and garnish should never overpower the main dish — they should work to accentuate and draw attention to the main dish, which is the subject of focus.
See also:

Why focus on the eye closest to the camera instead of the eye in the middle?
outdoor group photo - depth of field, other concerns?

Another way to look at it, is to consider the case of a wedding photographer. Most of the shots a wedding photographer takes is of two or more people. When there's a group shot, does the wedding photographer set their camera at the hyperfocal distance? Hell no. They focus on the person nearest the center of the group — usually the bride, then the groom, and everybody else is incidental.
Comment: The ratio of front:back DoF only approaches 1:1 at very close focus distances. For typical portrait distances, 1:2 (1/3 in front, 2/3 behind) is much more typical and what I tend to use when shooting group portraits. One must also consider that the human brain sees a group more in focus if those nearest the camera are at least as sharp as everything else. If the middle row is slightly sharper than the front row, it is very noticeable to the human vision system. If the front row is slightly sharper than the back row, not so much.
Comment: @MichaelC yeah. I decided to very much handwave the front:rear DoF ratio because I didn't want to get into those weeds. My main point was akin to a maxim from software engineering: "premature optimization is the root of all evil". Regarding your point about focusing on the front person, I  agree with that as well, and that was the point of the best answers of the first "see also" link.
Answer: Acceptable sharpness/depth of field (DOF)/apparent sharpness are all perceptual qualities dependent on how an image is viewed (size/distance). But actual sharpness/maximum sharpness only exists at the singular plane of focus. That may mean that those in front of/behind the plane of focus are "not as sharp as portraits should be," or it may not... what is the standard/requirement?
Hyperfocus is about maximizing the DOF by focusing at a specific distance based upon the effective focal length and aperture, and then everything w/in the scene just falls within the DOF to be acceptably sharp; rather than anything being actually in focus necessarily.
If you focus beyond the HFD you loose a little from the FG and there is nothing to be gained in the BG (already to infinity). And if you focus short of the HFD you gain a little in the FG but you loose a lot from the BG.
And hyperfocus is fairly pointless unless the lens is of a rather short focal length; ~≤ 50mm. For a lens set to f/16 on APS-C(1.5x), or f/11 on FF, the HFD in feet is ≈ to the FL as a percentage of itself... i.e. 50% of 50mm = 25ft and 10% of 10mm = 1ft. And if you halve the aperture # you double the HFD... i.e. 50ft for a 50mm @ f/8 on APS or f/5.6 FF. For a 100mm lens on APS @ f/16 the HFD would be ~100ft and result in a DOF from ~ 50ft to ∞... fairly useless IMO.
Added into all of this is the fact that, in practice, it's nearly impossible to determine the precise distance at which you are focusing... especially with short throw modern lenses.
In general, unless you absolutely need both very near and very far points acceptably sharp, then it is best to place the point of focus at the correct distance (on subject).
** all numbers are just a SWAG **
Answer: Focusing at the hyperfocal distance includes the concept of infinity, with the depth of field extending from infinity back to half of the hyperfocal distance. The sharpest point is always at the focus distance.  Infinity seems quite inappropriate for a group photo (Unless perhaps you are outside at a scenic view and do need to include infinity, but you might want to consider a closer background for a group portrait).
Depth of field at a specific focus distance has a near limit and a far limit of depth of field.  Go by that for a group portrait.


What to do with no stronger players around?

Question: I hear that a good way to improve is by playing against much stronger opponents. The problem here is that there are hardly any stronger opponents where I live and that makes it not only not as challenging but with no one to really personal help me improve. I thought of playing chess on the internet to resolve this problem but something about it just doesn't sit well with me, in other words I really dislike online chess. Maybe its the fact it doesn't feel like you are playing a real person or you're just staring at a screen instead of playing with a real board.
Comment: I know this is a silly suggestion, but concerning the part about staring at a screen instead of a real board, you could perhaps get a robotic arm to control your online opponent's pieces -
http://www.instructables.com/id/Chess-Robot/
Comment: If you're playing with a clock, you can use a time handicap
Comment: Build an A.I that can play chess something like deep blue. This will of course improve your chess and programming skills.
Comment: @HariPrasad But OP wants to avoid staring at a screen.
Answer: If there are no stronger players convenient to you, and you don't want to use the technology available to play with stronger players further away, then you must go to where the stronger players are.
Organizations exist to do just this.  For instance the US Chess Federation has tournaments around the US throughout the year that allow players to meet and play together.
Check out the organizations that serve you and see what opportunities they offer.  Some will require extensive travel, but with time and patience you will find chess clubs and tournaments that are within driving distance.
Once you establish a relationship with such a club or organization, and specific players within it, you may find playing online more comfortable once you know the people you are playing against.  Sometimes the issue isn't the mechanism, but the distance and relative anonymity.
Answer: Have you considered correspondence chess? You might have to use the internet for it but its fairly different from online chess if you ask me.
Answer: What type of online chess do you play? Blitz is not a good option, if you want to improve. If you want to play and improve using the internet, you should play standard time controls (at least more than 20 min/game). There are many good players on for example ICC and FICS playing slow time control chess. Try to chat with your opponents after the games, to make it more than just staring at a computer screen. Otherwise it is unfortunately hard to find strong players to play against if you live in a place without many chess players.
Answer: If you want to play with stronger players, you need to be where those players are. I see two major ways to do this:

Travel to where they are. You might only be able to do this every so often, but if a major tournament comes to your city or one within a few hours, maybe make a weekend of it and go to that tournament. You're likely to hit a bigger player pool at a bigger tournament, and usually they have brackets specifically for higher level players.
Bring them to you. Other high level players in the area may be in the same boat you are: they don't go to local clubs because there aren't enough (or any) strong players to challenge them. Try making a Craigslist ad that advertises your club and make sure to put a note in there that there's players of rating X that are seeking challengers. It might even be that a slightly lower ranked player that you're not used to playing will be a refreshing change, even if not as challenging.

There are other things you can do to keep your skills sharp. You can design puzzles for your club, and even post them online. Indeed you can become a titled Grandmaster just for making chess puzzles. You can also coach a chess team. This has the advantage of passing on your skills to a new generation of players while at the same time giving you a chance to meet potential challengers (the other coaches and enthusiasts at the event).
Answer: Online chess is probably the best answer, particularly if you don't have enough tournaments nearby (or can't afford to go to many). It can be made better than just sitting in front of a screen, though, for sure.
You can play it like I played correspondence chess back in the day (chess by mail): with a real board on your side.  Every move you make, make on the real board, then enter on the computer.  Every move your opponent makes on the computer, make on the real board.  That way it still has that feel.
As far as not having a real person sitting across: once you find some good players and play several games against them, you may find one or more are willing to do Skype/etc. games (where you play in a video call in real time, with a real board in front of you, and chat on the video call).
Answer: Teach them to become strong. This is what I did   with  my brother. And now he had became a tough competetant for me.


can this patent on smartphone app stop me?

Question: I developed a software system that comprises an app for smartphones, one for tablets and a central server that manages the communications between all the instances of them.
I did a research and found out that a company already requested to the PCT an international patent for a system that does exactly the same of what mine is intended to.
This patent has claims regarding what this system is used for (the IDEA), describing the architecture (central server connected to apps for smartphones and tablets and printers) and technology used for it (basic IP, XMPP, Push).
    Can this patent (I am not sure this is been approved or not) can actually causes me legal problems when I will sell my system?
I ask this since I don't see any technological innovation, only an innovative idea.
I intend to sell in the EU first and then worldwide (if it works of course!!!)
Thanks in advance for your help!
Comment: This does not sound like it should be granted a patent. You can try to stop the patent by sending in previous art.
Comment: Since you haven't provided the application number or the specifics of the application it isn't really possible to answer. "Sending in previous art" is advice specific to the US system. In the EU, if the PCT has or does enter the national stage in the EPO, you can use the pre grant opposition mechanism. It is not just sending in stuff.
Comment: Thanks for your comments.
@GeorgeWhite: here you have the link for the application request: [request](http://patentscope.wipo.int/search/en/detail.jsf?docId=WO2013061289&recNum=1&tab=PCTClaims&maxRec=&office=&prevFilter=&sortOption=&queryString=)
In my searches I found out that at the EPO it has not yet been granted a patent (status is A1), while at the USPTO I couldn't find any reference to this application.
Answer: An application is not a patent, so until granted there's nothing stopping you going to market, but there obviously could be issues down the line. As to the WIPO patent APPLICATION in question, the reason you can't find a USPTO reference is they haven't included the US in the list of: Designated States they are considering seeking protection in.
As to the EU, if you look at the status you'll see: "the application is deemed to be withdrawn" 2015-01-21


How to encrypt file with RSA and AES that anyone with RSA key can decrypt them?

Question: How to encrypt a file with RSA and AES-256 that anyone with RSA key can decrypt them?.
Comment: Well come to Crypto.StackExchange . Please post your question well-formated and well described.  Public key algorithm ( like RSA), in general, are used to key exchange or signatures whereas private key algorithms are used to encryption. So, what is your aim?
Comment: If you want to encrypt a file so that someone else can read it, you give them the key.
Comment: gpg would work (this uses RSA keys by default and can use AES-256 if so configured); anyone with the private RSA key can decrypt.
Answer: It sounds like you're looking for a hybrid crypto solution.  Some libraries provide APIs for this, e.g. libsodium.  Others, you have to build it yourself with the RSA and AES primitives provided, e.g. OpenSSL.
Without more information from you on your environment (e.g. programming language) it's really hard to provide any specific help.
Answer: Another name for hybrid crypto is envelope encryption. If you have a file you want to encrypt, you choose a random value to use as an AES key, encrypt the file using AES and that key, then encrypt the AES key using the RSA public key of each user to whom you want to grant access to the encrypted file.
Note that this is pretty much what GPG does if you encrypt a file to a list of public keys. The algorithms might not be AES and RSA, but they will be a symmetric encryption algorithm to actually encrypt the file, then an asymmetric encryption algorithm to encrypt the symmetric key to each public key.


Short link to read complete post

Question: I am just a newbie to the WordPress. I was just going through the demo sites and I found a website. Here you can see there are couple of posts in a home page and just below the every post there is a link.Here I want to know how all the posts are rendered in the home page and when someone clicks on the continue button how someone can read the complete post. So can some one kindly tell me that can be achieved. It may be a simple thing but as I am newbie to WordPress I really want to know how to do that. Thanks in advance.
Answer: they probably use the_excerpt(); on the front page. Read the whole thing here: http://codex.wordpress.org/Function_Reference/the_excerpt
Answer: You need to add the more tag in the editor when writing your posts. It looks like this http://alexmansfield.com/wpress/wp-content/uploads/2011/06/more.jpeg
Or you can add this  when editing the HTML.
Further information can be found here
http://codex.wordpress.org/Customizing_the_Read_More

Short link to read complete post


Laser beam welding

Question: I would like to use a laser beam to weld small metal parts that are located within an air tight enclosure but want the laser head to be located outside the enclosure which would require the laser beam to pass through a glass window. Can laser beams used to weld metals pass through glass?
Answer: Most weld optics have a protective glass that shields the (more expensive to replace) focusing lens from welding spatter. So there are definitely glasses that exist and can be used for this.
When designing your enclosure and/or the location of the metal parts during welding operation, try to let the laser beam propagate through the window as soon as possible after the focusing lens. The greater the distance, the higher the power density on the glass. Your design choices here of course depend on the focal length of the weld optic.
You might also need active cooling for the window, depending on the power of your laser.
The actual material depends on the wavelength of your laser and the power range, for simple (low power) applications, it is often BK7, for higher power quartz glass.
Important: Such a laser protective glass is a special optical component that is particularly picked for power and wavelength. Do not use a random piece of window glass or similar without checking the absorption, transmittance and reflectance of the material at that wavelength. You don't want to direct a 2 kW disk laser on your protective glass and have half of it reflected back into the resonator or even worse, into the room you are standing in.

Laser beam welding


Is a floating pin equivalent to a very weak pull down?

Question: I know that if a MCU pin is configured as "input" and it is not connected to any pull up/ pull down resistor, it is said to be floating.
In this situation, we could say that the pin is in series with the (very) high input impedance of the MCU, which is eventually connected to ground.
So we could say that the pin is in a (very) weak pull-down condition, with the MCU input impedance acting as the (very high) pull-down resistor.
So basing on this assumptions, if we suppose that no significant interference acts on the pin, I would say that (if we wait long enough) the pin voltage will be eventually "pulled to ground". Where am I going wrong?
Comment: Why have you chosen "ground" the MCU is also ultimately connected to VCC
Answer: Have recently been characterizing an input pin on a PIC microcontroller. The pin can be alternately set to CMOS in/out, Schmitt input, or analog input. When set for Schmitt input:

Data sheet specification of leakage current is +/- 5 nA.
Measured leakage current at room temperature is less than 3 pA.

Similar results were measured years ago on a different PIC microcontroller. Be aware that leakage current is very temperature-dependent, and can be in either direction. So you cannot assume that it floats to a logic low state.
It is possible that electric field of a hand-waving over a floating pin can change its logic state.
Answer: A CMOS chip's input circuit generally includes both P channel and N channel transistors.  If the chip is powered, then the (tiny) gate leakage currents can be in either (or both) directions: to ground, or to some internal voltage rail(s).  The process variation may not allow one to pre-determine which direction of leakage is greater (unless the chip was designed with specific direction and process+operating margin).
Answer: tl; dr version: The input impedance of a CMOS input is high, practically an open circuit. It needs to be terminated, either by tying to a supply or by driving it with a signal. It cannot be left floating.
Why?
A CMOS input is a capacitance: the gate-source capacitance on the input pair. Your intuition tells you that even though it is a capacitance, the charge on the input should bleed off somehow over time. In fact CMOS gates do that: they have a small Vgs leakage. There is also leakage in the input protection circuit - a pair of reverse-biased diodes that catch voltage above VDD and below Vss.
However, there's two confounding things going on. First, the two FETs (P and N) have their sources tied to the Vdd and VSS rails, respectively (as do the protection diodes.) So their leak paths are to opposite rails: P FET to Vdd, N FET to Vss.
If the leakages were equal, then in theory the input would find a bias point in between. In reality this is very hard to predict because gate leakage varies wildly with process and temperature. It cannot be counted on as a bias.
Second, this Vgs gate leakage is very, very small, and so is the gate capacitance (a few pF typically.) On the other hand, the influence of outside electric fields can easily couple onto the floating pin and cause it to change state. This is in fact how a basic capacitive touch sensor works: hum pickup from the body.
Bottom line: never leave a CMOS input floating. Always tie it off.
Answer:

In this situation, we could say that the pin is in series with the (very) high input impedance of the MCU, which is eventually connected to ground.
Where am I going wrong?

You are going wrong in forgetting it is also eventually connected to Vdd, not just GND. So why would it eventually pull to GND over Vdd?

simulate this circuit – Schematic created using CircuitLab
Answer:
Where am I going wrong?

...to the fact that a pin can be connected to two CMOS in push-pull configuration, and they could leak current in an unpredictable way...
Thinking better, even if the pin is a pure CMOS input, you can not know in advance what will happen. May be that for a short time our CMOS input behaves like a load.
A (non-CMOS) transistor can be very different, but if you restrict the question to MCUs, I believe it is difficult to find normal transistors.
Answer: No, a floating pin cannot be considered to be equivalent to having a defined pull-down. It could also tend to float high or at midpoint or any level.


Are there any natural sweeteners that work well with coffee?

Question: I am curious about different natural sweetener options that might mix well with coffee. I know about stevia but it gives the coffee a different flavor. Is there anything besides stevia that I'm not aware of? My apologies, I meant natural non-sugar alternatives, I should have specified that.
Comment: Do you not want to use actual *sugar*? This is perhaps the most "natural" and common natural sweetener that there is. What are your criteria for selecting a sweetener -- naturally-produced, non-nutritive, minimal off-taste, ...?
Answer: The best natural options for sweetening are, in my humble opinion, actual sugar or sugar syrups. Options include, in general order from less to more distinct taste...

table sugar (sucrose), including common refined white sugar or less-refined options like evaporated cane juice or turbinado;
honey;
agave syrup;
jaggery or similar minimally-refined sugar;
malt syrup, from e.g., rice or barley;
maple syrup or maple sugar;
molasses.

Though I personally prefer my coffee black and unsweetened, I've used all of these in coffee. Any sweetener will change the taste slightly. The problem with stevia, for me anyway, is that the bitter notes of stevia are overpowering; I can't consume the stuff.
If you're looking for a non-nutritive or lower-calorie alternative, you can look at various sugar alcohols, which are (or at least can be) naturally-produced, lower-calorie variants of sugars. They are produced by various methods, including fermentation.
A good possibility is xylitol, which is somewhat lower in effective calories and nearly as sweet as table sugar with less off-tastes than other sugar alternatives. Another option is erythritol, but may be less-commonly available. World-wide, other options including mannitol, sorbitol, maltitol, and isomalt may be more common.
Note well: different sugar alcohols may have effects on your digestive system, including bloating and loose stool, especially when consumed in large quantities. Start slowly :)
Comment: If this answered your question, consider upvoting and marking as the answer.
Answer: Xylitol works really well as an alternative to sugar. I've made two coffee's, one with sugar and one with Xylitol, to taste test, and could only just tell the difference. No funny after tastes like many other alternative sweeteners.
Comment: where do you get xylitol?
Comment: @AdaLovelace Depends what part of the planet you live on, I imagine. In the Antipodes you can get it as health food stores, where it comes under various brand names (e.g., Perfect Sweet). Also, a search on Amazon brings up a bunch of options.
Comment: I live in US. I checked Amazon, and there are a ton of options.
Comment: Just a note, xylitol is deadly for dogs, so keep it away from them.
Answer: Here is one Monkfruit! I find this sugar replacement to be very tasty without the negatives of stevia. It's expensive, but one of the best options if you are looking to be healthy.
Lakanto Monkfruit Sweetener
Answer: Honey works the best. It smoothes taste and adds mild sweetness to your cup.
For the best experience and authentic taste please do not use generic pre-heated honey and go for organic already crystallised honey.


REST API & NPSP error: FIELD_CUSTOM_VALIDATION_EXCEPTION

Question: I have NPSP v3 set up in Salesforce and am using the REST API to create a contact record. I am inserting fields [FirstName], [LastName], [Email], [Phone], [MailingState], [MailingPostalCode], [RecordTypeId] only.
It works fine but I have a case that is throwing the error

Error: call to URL
  https://axysau.my.salesforce.com/services/data/v20.0/sobjects/Contact/
  failed with status 400, response [{"message":"Failed to create Account
  for Contact FIRSTNAME LASTNAME. You are creating a duplicate record.
  We recommend you use an existing record
  instead.","errorCode":"FIELD_CUSTOM_VALIDATION_EXCEPTION","fields":[]}],
  curl_error , curl_errno 0

It is failing for a case where the contact is already in Salesforce with a different email address. I have checked validation rules on Account, Contact and npsp__Address__c but none of them have any. I also checked 'Household Account Addresses Disabled' in NPSP Settings->People->Addresses. I still get the same error.
So I changed the postal code for that contact before its created with the API and it has successfully created the Contact & Account for them.
Where can I change settings so that contact with the same firstname, lastname & postal code as someone already in Salesforce doesn't block me from creating a new Contact & Account record for them? No doubt I will get the same people using a different email address, or different people with the same name living in the same suburb. I want it to create a new record in all cases.
Answer: This is caused by the Duplicate Rules feature.  The error you're seeing is deceptive because the Duplicate Rule error is typically DUPLICATES_DETECTED, Use one of these records.
Check Setup to see if you have Duplicate Rules configured.  If you have active rules that are set to Allow, then go to the NPSP Settings tab > System Tools > Error Notifications and check the Respect Duplicate Rule Settings.
Comment: Thank you! I've disabled the Duplicate Rule on the Account object and I can successfully add the record now. I'll see about re-activating it and modifying the rules.
Answer: The problem is not Validation Rules but Duplicate Rules.
NPSP is trying to create a Household Account for each Contact you insert. However, the Account creation is being blocked by the Duplicate Rule on Account, likely meaning that you already have a Household Account by that name. You may wish to customize Duplicate Management to change the criteria used to identify duplicate records to be more rigorous, or even deactivate it entirely.
Comment: Thank you! I've disabled the Duplicate Rule on the Account object and I can successfully add the record now. I'll see about re-activating it and modifying the rules as you've specified.


Would a RAM upgrade do anything

Question: I'm going to be fixing up an old box I had in the closet and running debian on it, going to be an all-purpose server for my own uses... but I wouldn't mind it being a game server or something... if it's got a P4 @ 1.8 GHz and 512MB RAM, would throwing a extra 1GB in it help any?
Answer: Yes, depending on the operating system you are running it might help more or less (Debian from my experience is not that resource hungry). Any time you increase the amount of physical memory in a computer, you should notice a significant performance gain.  The performance gain will be less noticeable when you are running fewer applications, but will be dramatic under heavy usage. Just make sure you're motherboard supports the memory upgrade before you commit to buying anything. Some motherboards limit the amount of RAM per slot/Total per board, and place other such restrictions on adding RAM.

Would a RAM upgrade do anything