Multi-scale in Object detection?

Question: I want to ask about MultiScale Object Detection. For the multiscale, if we have an original image do I need to zoom-in at different scales in the image at a particular point or zoom-out at different scales in the image at a particular point?
Answer: Multiscale object detection generally consists in smoothing the image with larger and larger kernels, to check  which objects persist across different scales. So, zoom-out, mostly. You now can have access to the scale-space representation of the image, as shown below.

One  reason is that, without more knowledge, one does not zoom in beyond the pixel width. As illustrated (with humour) in the following picture.

Meanwhile, in certain conditions, subpixel resolution or superresolution is possible.
Comment: For the first image, the size of the image is same its just the filter size is increasing I guess. but is it same if we use MATLAB command imresize and downsample the image from 720x940 to 256x256 and then further 128x128 and 64x64?
Comment: `imresize` and `downsample` provide instanced in the large class of scale-space/multiscale techniques. Using the low-pass effect of larger kernels, you can indeed downsample the low-pass images, and obtain something akin to the Gaussian pyramid. YOu can find a description of the above in the survey paper [A Panorama on Multiscale Geometric Representations](https://arxiv.org/abs/1101.5320)

Multi-scale in Object detection?


Find Raspberry PI address on local network

Question: I need to find the Raspberry PI IP address on local network without connect the Raspberry to a monitor. How do I do it on Linux or Mac OS systems?
Comment: possible duplicate of [SSH into raspberry pi without knowing ip address](http://raspberrypi.stackexchange.com/questions/12440/ssh-into-raspberry-pi-without-knowing-ip-address)
Comment: Lan scan app on smart phone will show all devices on your network.
Answer: All raspberry devices MAC addresses started with B8:27:EB (for RPi4 B they start with dc:a6:32, replace where appropriate).
So, on *nix systems, this can be accomplished by executing the following command:
nmap -sP 192.168.1.0/24 | awk '/^Nmap/{ip=$NF}/B8:27:EB/{print ip}'

where 192.168.1.* will be your local network mask. You will get an answer like:
Nmap scan report for raspberrypi.localnetwork.lan (192.168.1.179)

The 192.168.1.179 is the Raspberry Pi IP address on you network.
If you are on a Mac system you can use Homebrew to install nmap:
brew install nmap

On Windows, you might use Wireshark and use the following display filter:
eth.addr[0:3] == B8:27:EB

Comment: You would not appreciate the importance of this answer until you have tried to headlessly locate your raspberrypi in an office setting where IP's are dished out using DHCP. Cheers for this @ripat
Comment: This answer didn't work for me since nmap wasn't spitting out the hostnames for me.  However, it did inspire me to search for open ssh ports with nmap and that did enable me to find the pi: `sudo nmap -p 22 --open -sV 192.168.0.0/24`  [per this guy's post](http://thoughtsbyclayg.blogspot.com/2008/06/use-nmap-to-scan-for-ssh-servers-on.html).
Comment: Or with less pipes: `nmap -sP 192.168.1.0/24 | awk '/^Nmap/{ip=$NF}/B8:27:EB/{print ip}'`
Comment: Thanks @ripat, that is a lot more concise - updated! One little detail, you need to run as administrator or the nmap will not display the MAC address.
Comment: An improvement to automatically grab default IP and subnet mask, and also display hostname and MAC in addition to IP: ``sudo nmap -sP `ip -o -f inet addr show | grep \`ip route get 1 | awk '{print $NF;exit}'\` | awk '{print $4}'` | awk '/^Nmap/{ip=gensub(/[^0-9\.]/,"","g", $NF);host=$(NF-1)}/B8:27:EB/{printf "%s\t%s\t%s\n", ip, $3, host}'``
Comment: Addendum: For RPi4 B they start with `dc:a6:32`
Comment: isn't "sudo nmap -sP 192.168.1.0/24" enough (it wored for me)?
Comment: also on mac os x, you can install nmap using fink
Comment: Tried it but it couldn't find my rasp 4
Comment: Some RPi4 start with `E4:5F:01` as well: https://udger.com/resources/mac-address-vendor-detail?name=raspberry_pi_foundation
Comment: For me, only @RossRogers command worked. The accepted answer did not find my RPi that I had freshly installed and activated ssh.
Comment: And ChatGPT just helped me to make it greppable: `nmap -p 22 --script=banner -sV -oG 192.168.0.0/24 | awk '/Rasp/ {print $2}'`.
Answer: If you're running MacOS or Linux, try this one:
$ arp -na | grep -i b8:27:eb

On windows, you can use:
arp -a | findstr b8-27-eb

On RPi 4B, you need to use different string for comparision (dc:a6:32:be:95). You can check it on your RPi by
ip -br link

There will be entries for each network card. eth0 is cable internet, wlan0 is the WiFi.
Comment: This will only show hosts which have made it into your arp tables, so sometimes misses the quiet ones.  It might help to do an nmap scan once first.
Comment: This solution is much faster than an nmap solution!
Comment: It might, but for me this never failed. Plus it's an easy first try.
Comment: This solution worked for me, unlike the nmap solution which gave only part of the list.
Comment: Thanks for this. I modified it slightly and use `arp -n | awk '/b8:27:eb/ {print $1}'` now.
Comment: not working for me if already not tried connect to device
Answer: I use mDNS/Bonjour to give my Pi a local name like pi.local. I dont need to know it's IP address and this works for shortcuts and when you need to embed a link...
How and Why to assign a local name to your Pi
Comment: Awesome! I used to fiddle around with `/etc/hosts` locally to get this functionality on my desktop, this is a MUCH better solution! Thanks so much for the pointer!
Answer: By default the raspberry's name is raspberrypi. You can just 'ping raspberrypi' and the ICMP echo reply gives you the IP address. It worked for me. Make sure though the DHCP server is reachable as the raspberry's NIC is by default in DHCP client mode. Otherwise the raspberry gets an APIPA address.
Comment: I guess this only works if you already have a shell on the Raspberry Pi, which is not what was asked.
Comment: trying to figure out how and by what service "raspberrypi" is resolved to an IP? As it is not in my host file, nor my dns?...
Answer: If you want to use a GUI application for it, you can use Yakala tool (https://github.com/mozcelikors/yakala) for Debian/Ubuntu. This tool helps you not only search for available Type B and Type C networks, but also helps you to directly SSH into the network:
sudo add-apt-repository ppa:mozcelikors/yakala
sudo apt-get update
sudo apt-get install yakala

or
git clone https://github.com/mozcelikors/yakala
cd yakala && sudo ./install.sh

Disclaimer: I am the creator of Yakala. Any suggestions/bugs are taken very seriously.
Comment: Classful networking was abandoned in 1993, which is a very long time ago.
Comment: What is classfull networking?
Answer: I use the free Fing app in my android smartphone. It scans the network and shows the connected devices by type, including Raspberry Pi, as well as scanning available ports. Handy to see if SSH, web or VNC are enabled and running.
Answer: In 2020, the accepted answer does not work anymore because the Raspberry Pi ships with a different range of MAC addresses. I could just add the "new" MAC as a comment, but the string could change again and again each model. Here is something that's more future-proof.
$ sudo nmap -sP 192.168.150.0/24 |grep 'Raspberry Pi Trading' -B2
Nmap scan report for pi4-01.lan (192.168.150.186)
Host is up (0.037s latency).
MAC Address: DC:A6:32:1B:35:6A (Raspberry Pi Trading)

The main difference above: we don't assume a MAC pattern, we just grep for the (summary), as this label is provided by nmap itself.  Because nmap is continually updated, it contains internal tables of Raspberry Pi MAC addresses (or uses other criteria to detect) and we can assume that will continue working.
CAVEAT: If your scan does not identify all Pis you know are on the network, then repeat the scan -- up to 10 times. A Pi which has networking Power Management:on might not always respond to a scan.
Power Management may be a problem for you if you are scanning for multiple Pis on a LAN, as repeat scans might detect one Pi but not another. In that case, repeat the scans look for differences. (To confirm if a Pi has Power Management enabled, ssh to the Pi and run: /sbin/iwconfig wlan0|grep Management)
Answer: In linux and MAC, you can also type in "arp - a" in the terminal and you can get a list of connected devices, look for the one with B8 in it, example: 192.168.4.5 @ B8... will be the raspberry pi IP.
Answer: If you run MacOS, use PiFinder, it is an application that will tell you the IP of a Raspberry Pi on your network.
Answer: Or you could access to your Router via browser and find your android device -almost every router GUI has a service where you can check all devices that are currently connected to your network.
Answer: Copy the following into your Terminal/Console/Shell:
for i in $(jot - 1 254); do ping -t 1 192.168.1.$i && arp -a | cut -f 2,4 -d " " | tr [:lower:] [:upper:] | grep B8:27:EB; done

Please adapt 192.168.1. to your individual network.
The output will look something like this:

(192.168.1.109) B8:27:EE:DD:CC:A

192.168.1.109 would be your Raspberry Pi's IP… :)
Answer: When using a fresh install of NOOBS or Raspbian, the default hostname is "raspberrypi" and in some network configurations (i.e. if mDNS is in use and configured) you could use "raspberrypi.local".  Another option is to run arp -a and look for raspberrypi
NOTE: This won't work for every instance.
Comment: That answer seems to mix up several things: different IP versions, DNS, and maybe mDNS. Whether `.local` can be used usually doesn't depend on IPv6, and also not on having a router present. When mDNS support is available then pi-name.local resolves, IPv6 or not. Some home routers also use .local as their local DNS domain, again this is independent of IPv6 and depends on the router model. Some home routers use different domains, such as pi-name.fritz.box.
Answer: Put this in pi.php on your web server:
<?php
$fp = fopen('pi', 'w');
fwrite($fp, $_REQUEST['ip'],1000);
fclose($fp);
?>

You may have to create the file pi on your web server with write permissions for your web server.
Put
curl http://yourwebserver/pi.php -d ip=`hostname -I`

in /etc/rc.local on your pi.
Get the IP of your pi by browsing http://yourwebserver/pi
Answer: My router assigns newly discovered systems an ip-address starting at 192.168.1.2, sequentially. I went through every system in my home one day and reserved addresses for them.
That way when I create a new system, chances are that its ip-address will be 184.108.40.206.
Another way to use the serial console. You can use a serial terminal program in order to log into your raspberry pi. I would suggest using the 8N1 (8-bits, no parity bit, 1 stop bit) setting with a speed (baud rate) of 115,200. You'll need a USB to serial (3V3) connector.
Answer: 'Fing' or 'Net Analyzer' on Android. Shows your whole network.
Answer: If you are on Windows I suggest you to try out this script.
It is mostly based on the "arp -a" command, but it saves you some extra digging and helps you when your Raspberry Pi is not already in the Arp table.
Create a text file with the following content and rename it as find_raspberry_pi.bat
@echo off
:: This script, run from a Windows (10) machine, finds a Raspberry Pi on local network

:: You can manually set subnet if needed
:: E.g.
::SET subnet=192.168.56
SET subnet=

:: --- No need to edit below this line ---

SET raspIp=
SET myip=

:: Get IP for local PC
FOR /f "tokens=1-2 delims=:" %%a IN ('ipconfig^|find "IPv4"') DO SET myip=%%b
SET myip=%myip:~1%
echo IP for local PC: %myip%

:: Retrieve subnet if not manually set
IF "%subnet%" == "" (
    SETLOCAL EnableDelayedExpansion
    SET subnet=%myip%
    FOR /l %%a IN (1,1,31) DO IF NOT "!subnet:~-1!"=="." SET subnet=!subnet:~0,-1!
    SET subnet=!subnet:~0,-1!
    SETLOCAL DisableDelayedExpansion
)

:: Show subnet
echo Subnet: %subnet%
echo.

:: Loop through arp table entries and look for Raspberry Pis MAC addresses
echo Discovering network...
:: Ping all IPs in subnet from 1 to 254
FOR /L %%N IN (1,1,254) DO start /b ping -n 1 -w 200 %subnet%.%%N >nul
timeout 1 >nul
FOR /f "tokens=1" %%f  IN ('arp -a ^| findstr "28-cd-c1 b8-27-eb e4-5f-01 dc-26-32"') DO (
    IF NOT "%%f"=="" echo Found Raspberry Pi having IP %%f
)
echo.
pause

Disclaimers:

I am not the author of the original script, I just adapted it a little bit to work with a Raspberry Pi. Sadly, I cannot find the original contributor.
It's not bullet-proof, it will not work if the subnet it starts scanning is not the correct one if you want to scan a different subnet you need to manually edit the appropriate line at the beginning of the script.

Still, I used it many times and it's a simple solution.

UPDATE 2022:
I updated this script. Now:

it can find multiple Raspberry Pis at once
it can find allegedly all types of Raspberry Pis (3, 4, etc.)
you can manually force a specific subnet if the autorecognize feature finds the wrong one
Answer: Raspbian is using a thing called mDNS, which allows you to access your raspberry pi by a hostname raspberrypi.local.
So, you don't need to search for an IP address anymore, just connect like so:
ssh firstname.lastname@example.com

Answer: Try this first:
arp -a | grep -E --ignore-case 'b8:27:eb|dc:a6:32'

The two hex strings (b8:27:eb|dc:a6:32) in this command reflect the two OUI values used by "The Foundation" for production of all RPi devices - through RPi ver 4B as of this writing. If your RPi isn't in your arp cache this command won't yield anything useful. If that's the case, then create the following file in your favorite editor, and save/write it as pingpong.sh:
#!/bin/sh

: ${1?"Usage: $0 ip subnet to scan. eg '192.168.1.'"}

subnet=$1
for addr in `seq 0 1 255 `; do
( ping -c 3 -t 5 $subnet$addr > /dev/null ) &
done
arp -a | grep -E --ignore-case 'b8:27:eb|dc:a6:32'

make it executable, and run it (use your network address here, not necessarily 192.168.1.):
$ chmod 755 pingpong.sh
$ ./pingpong.sh 192.168.1.

Your results may appear as follows:
raspberrypi3b.local (192.168.1.131) at b8:27:eb:1:2:3 on en0 ifscope [ethernet]
raspberrypi4b.local (192.168.1.184) at dc:a6:32:2:3:4 on en0 ifscope [ethernet]

In this case, the hostname appears courtesy of mDNS. If you don't have/don't use mDNS/avahi/etc the hostname will be replaced by the character: ?.
Comment: This works great, thank you for sharing,
For me, however, on an M1, `arp` is not able to obtain any of the hostnames, I think it could be because `/etc/resolv.conf` is not used in my system ( not sure if that's just M1s or all MACs )
Answer: I assume you cannot connect to you PI, otherwise you'd know its IP.
If you go to your router page (usually something like 192.168.1.1 in your browser) there you should have a section with the already connected devices. You can try to plug and unplug the PI (and one device should appear / dissapear from the list)
Answer: I wrote myself a bash script to find my OctoPi a while ago.
You can change the systype="Raspberry Pi" to whatever type of device you are looking for.  It finds the subnet itself so you need not guess 192.168.1.0 or whatever.
There are now 3 different OUI ranges for the Pi Foundation:
https://udger.com/resources/mac-address-vendor-detail?name=raspberry_pi_foundation
B8:27:EB:xx:xx:xx   B8-27-EB-xx-xx-xx   B827.EBxx.xxxx
DC:A6:32:xx:xx:xx   DC-A6-32-xx-xx-xx   DCA6.32xx.xxxx
E4:5F:01:xx:xx:xx   E4-5F-01-xx-xx-xx   E45F.01xx.xxxx

So the ping grep answers need to add those
[andy@localhost ~]$ sudo ./findpi.sh

Searching 192.168.68.0/24 for Raspberry Pi systems
My address is wlp0s20f3:192.168.68.128

Found 1 Raspberry Pi system
    Raspberry Pi system found at 192.168.68.124

I fixed it up a little and here it is:
#!/bin/bash
#-----------------------------------------------------------------#
#  Find PI bash script
#   Andy Richter
#   05/19/2021  I really wrote it in 2019 when I got my printer
#  This script will search the local subnet for Raspberry Pi systems
#  The script requires root access for nmap
#
#   There are no parms we find things for ourselves
#------------------------------------------------------------------#
systype="Raspberry Pi"
#systype=Unknown
me=$(readlink --canonicalize --no-newline $0)
if [ "$EUID" -ne 0 ]
  then echo >&2 "Please run as root or use sudo $me"
  exit
else
  if ! command -v nmap &> /dev/null
    then
       echo "nmap not found on this system. Please install. exiting"
       exit
  fi
fi
# awk doesn't like variables between /.../
awkstr="/^Nmap/{ipaddress=\$NF}/$systype/{print ipaddress}"

# Use ip route to get the active rout to the internet, IP address and interface
#  Then use that interface to find the subnet
myrout=`ip route get 126.96.36.199`
myaddr=`echo $myrout | grep src| sed 's/.*src \(.* \)/\1/g'|cut -f1 -d ' '`
mydev=`echo $myrout | grep dev| sed 's/.*dev \(.* \)/\1/g'|cut -f1 -d ' '`
# now get our subnet
mynet=`ip -o -f inet addr show $mydev | awk '/scope global/{sub(/[^.]+\//,"0/",$4);print $4}'`

printf "\nSearching $mynet for $systype systems\nMy address is $mydev:$myaddr\n"

# Use nmap to find PI systems. 'Raspberry Pi' is found in NMAP for PI systems
foundSys=`sudo nmap -sP $mynet | awk "$awkstr"`
foundPI=`echo $foundSys`
numpi=`echo "$foundPI" | awk '{print NF}'`
s=$([[ ($numpi == 1) ]] && echo " " || echo "s")
# Now we know, print the list if any

printf "\nFound $numpi $systype system$s\n"
for ipaddr in $foundPI
   do
     printf "\t$systype system found at $ipaddr\n"
   done

exit 0

Nmap output looks like:
Starting Nmap 7.70 ( https://nmap.org ) at 2021-05-01 15:57 EDT
Nmap scan report for 192.168.68.124
Host is up (0.021s latency).
MAC Address: B8:27:EB:58:CA:8F (Raspberry Pi Foundation)


Why do users search for URLs?

Question: I've been reviewing my company's analytics recently, and a lot of people are using search engines to search for the company URL, or URLs within the company. It probably equates to less than 1% of search terms, but it's still more than I would expect, particularly as it slows down access to sites. Arguably browsers that allow searching from the address bar have blurred the boundaries between searching for content and going there directly, but I'd like to know if anyone has done any research into why people do this.
Comment: Peter, if you want micap's attention, you should prefix his username with an @ sign. He will then be notified in his inbox. I didn't have to do this for you, because post owners always get notified of new comments to their post.
Comment: Have you done any research into this already, or is this just idle curiosity?
Comment: A lot of the information that searching produces is around SEO and other searching behaviours. Not yet seen anything that explores the 'why'.
Comment: What did you mean by 'particularly as it slows down access to sites'?
Comment: micap - because it takes longer to type the URL into a search bar, wait for the results to come back, scan for the expected result, click on the link and go to the page than it does to type the URL into the address bar.
Comment: I know I have tried to type an URL into the location bar and ended up on Google's result screen many times. It happens. I wouldn't worry too much about it.
Comment: @Peter Normally, I think the URLs are _pasted_, rather than typed.
Answer: Many people have only a fuzzy awareness of the distinction between "The Web" and Google / Yahoo / whatever their homepage is set to. Therefore, such people often use the Google search box (or whatever) to enter URLs rather than the browser address bar.
Google may have noticed this behaviour when developing the combined URL/search box in Chrome.
Comment: That, but I also catch myself typing stuff in whatever white rectangle I can find somewhere at the top of the screen. Muscle memory is fast, but also inaccurate. Luckily these days it doesn't matter: Google will figure it out for you.
Answer: Consider what happens if they enter it incorrectly.
Address bar: nasty error message, have to try again
Search engine: "Did you mean..." and often a single click to get what they want
Comment: +1 correct. In many cases it takes less effort to search for a partial URL and select from the results than to type in a complete (and error free) URL.
Comment: I would add that sometimes your typing and it's not typing in the field you wanted and you typed enter faster than the chance you had to correct the mistake and said oh well.
Answer: I have observed this on a number of occasions and I believe people are confused because:
a) They use a search engine as their default homepage
b) A search box is visually similar to an address bar
c) Google (and others) auto-focus on their search box when visited
More importantly, less computer-savvy people look down at their keyboards when typing, especially for long URLs, and miss out on the fact that they are actually typing it into a search box.
Furthermore, since modern browsers use combined address bars and search boxes, this makes the distinction hard.
Answer: I think this depends on whether these are new or returning users. You may be able to see a correlation between the % of new users a month with the % of users who arrive from search.
Your analytics should also provide the graph of web browsers which could lay further credit to the other answers around user behaviour with omnibox.
You may find that there are a combination of reasons why users would rather search than type in the url.
Personally, I use sites which are following the current trend of not spelling their names correctly e.g. tumblr, toggl, dribbble so I've learnt to let Google handle the telling me where I want to go

Why do users search for URLs?


Why do airplanes use an axial flow jet engine instead of a more compact centrifugal jet engine?

Question: Axial flow jet engines take up a lot of space. Centrifugal jet engines create high pressure. Can you create a centrifugal air pump that pushes as much air as a axial jet engine. Centrifugal air engines reach higher speeds from my research. What are the main disadvantages of centrifugal air engines from the standard axial engines.
Answer: Centrifugal compressors only produce a more compact engine at low mass flow, which means low thrust.
The amount of thrust an engine can produce is proportional to its intake area times exhaust velocity. Increasing the latter is undesirable, as energy and thus fuel consumption is proportional to velocity squared. So engine designers target mass flow to gain more thrust, not more velocity.
Since they are 3D structures, in a basic solid design (that you'd find in early jets and small modern turbines), the volume of a centrifugal compressor grows in cubic proportion to its diameter, while frontal area, which limits its mass flow and thus its thrust, increases only as diameter squared. This creates a cube-square law.
Large real-life parts are filled with lightening and cooling channels, so the mass-to-area law is more complex. Still, it cannot eliminate the volume effect entirely. The end result is that the mass of centrifugal compressors grows considerably faster than their mass flow.
At the highest power levels, centrifugal compressors become prohibitively heavy even for fixed powerplant machinery, where durability otherwise trumps weight, so the largest base load powerplants run all-axial. With axial compressors, the flat design with a short air flow path allows mass to only grow in direct proportion to mass flow, and very powerful engines can be built within reasonable dimensions.
It's not pure cube-square law in either case, but it's something like k1*massflow^[2.5, 2.8] for centrifugal versus k2*massflow^[2.2, 2.4] for axial, where k2>k1, giving centrifugal compressors some advantage in small sizes. Centrifugals are also much cheaper to produce (at least small ones).
High-performance engines, such as those in commercial aircraft, also need to pack more thrust into the smallest cross-section they can, while maintaining efficiency, so as to reduce drag and also fit under the wings, enabling heavier jets. Axial compressors offer a lot more intake area for any given cross-section - thus more thrust.
The smallest jets, where thrust requirements are small and the engine's cross-section is very small compared to the fuselage, can afford the extra diameter of a centrifugal or diagonal flow compressor. Yes, it's the same cube-square law (also reduced somewhat in practice) that keeps the engine cross-section to total cross-section ratio increasing as aircraft go up in size. Small centrifugal compressors are simpler, easier to build, and more robust than small axial ones.
So in every industry, as power grows, there is a crossover point from centrifugal to axial. For aircraft where drag is critical, it's just above small bizjets, mobile ground and helicopter turbines stay centrifugal or mixed up to a few MW, and in the tens of megawatts even fixed powerplants switch from axial/centrifugal to all-axial.
Engines close to that crossover point typically combine axial and centrifugal stages. Newer diagonal compressors are in-between and quite good, offering an even more tailored compromise.
Comment: Thank you, a very thorough yet easy to follow answer.
Comment: "The necessary mass of a centrifugal compressor grows in cubic proportion to its diameter" Not necessarily: The length of an engine might even shrink when increasing the diameter. In this case the volume (and therefore the weight) would grow less than squared.
Comment: "The mass question is really quite complex." Exactly my point, yet you present it as a square-cube law.
Comment: "Centrifugal compressors are only more compact at low mass flow" no they aren't, they are more compact at any mass flow. A large part of the frontal area cannot be used for airflow, is the issue in larger engines.
Comment: @Koyovis Define "compact" then. Inlets have to be sized for required airflow, so, for airliner engines that are only as wide as their inlets, a wider compressor will mean a wider and heavier engine. Centrifugal compressors are compact in smaller engines, but supersizing them doesn't maintain that - axials are better at ingesting as much air as possible in given engine size.
Comment: Mass of the centrifugal compressor grows exponentially compared to the mass of three axial stages? The centrifugal compressor can be hollowed out, the main stress bearing material is at the impeller tip.
Comment: @Koyovis The mass question is really quite complex. You can hollow out more material from a larger impeller, but there's more centripetal force from the heavy tips. Overall the end result is that the largest centrifugal compressors are much heavier than their axial counterparts, while for small compressors all three solutions are mass-competitive.
Comment: @Koyovis Well, saying "it's complex" largely equates to "it's magic", so I went for a simplification. That simplification still holds true in a naive case, and for real-life scenarios, it correlates with them. I'll edit to clarify that.
Answer: Axial turbine engines take up a lot of space...lengthwise. Centrifugal compressors are shorter and wider, and are very often used in turboprop and turboshaft engines, for instance the Rolls Royce Dart is a single axis turboprop engine. The photo demonstrates the compactness of the engine...lengthwise.
.

Can you create a centrifugal air pump that pushes as much air as a axial jet engine.

Yes you can. It will have two main issues:

The losses in the centrifugal compressors will be higher than for an axial configuration reaching the same compression ratio.
Part of the frontal area cannot be used for airflow.

What are the main disadvantages of centrifugal air engines from the standard axial engines.

Centrifugal compressors:

Are slightly less efficient than axial compressors (for a given compression ratio).
Expel the airflow perpendicular to free stream, so that if multiple stages are required the airflow must be guided into quite a bendy pathway, with additional negative impact on efficiency.
Result in an engine with a larger frontal area.

Advantages are:

Centrifugal compressors achieve higher compression ratios than axial compressors - per stage. Compression ratios of 4 - 6, while an axial stage can only do 1.4 - 1.6.
They are of a more robust and often less costly construction.

Larger engines (high mass flow, high compression ratio and minimal frontal area) use axial compressors almost exclusively: the internal losses are lowest. Centrifugal compressors are used for design cases where other factors than efficiency are of interest, such as cost and limitation of length for helicopter turboshafts.
An example of the use of a centrifugal compressor in a turbofan engine is the Garrett AiResearch ATF3. A 3-shaft engine, with the fan on shaft 1, five axial stages on shaft 2, and a centrifugal stage on shaft 3.

Picture source
The hot exhaust stream is deflected back into the fan bypass, which cools it down and results in a low IR signature. This engine is used in the Dassault Falcon. Note that there is no principal technical difficulty to scale this engine up to A380 level, other than the decreased efficiency due to the multiple deflections in the airstream. The wider frontal area is not a problem in the compressor part, and the total compression ratio increases a lot using this final stage.
As in the ATF3, centrifugal compressors are often combined with an axial stage, which pre-whirls the air stream into the centrifugal compressor and increases both efficiency and max compression ratio per single stage, as compared to a flat single stage.
Answer: While a centrifugal compressor can acheive greater compression than a single compressor stage in an axial flow engine, the axial flow design allows for multiple compressor stages, achieving higher overall compression of the air and consequently greater efficiency. Centrifugal flow jet engines are, however, tough and reliable.
Comment: Centrifugal compressors can also be multi-staged.
Answer: Centrifugal compressors are limited by the size and sharp angle of the diffuser duct that has to straighten out the outward flowing air flow from the impeller compressor and reroute it back to the combustion chambers. This restricts the amount of air that can flow through this engine. Axial flow engines are straight through design and don't have this limitation.
Comment: You may expand your answer so that it is clear you add something to existing answers
Answer: All existing centrifugal-compressor aircraft engines have been turbojets or turboprops.
Aside from their low power compared with modern turbofans (the Rolls-Royce Dart mentioned in another answer produced about 1 MW, compared with 50MW for a modern large turbofan engine core) the larger diameter of a centrifugal compressor would make it hard to design an efficient turbofan, by restricting the area of the fan duct.
Increasing the outer diameter of the fan blades to enlarge the bypass duct is not a practical option, since it is limited by the ground clearance of the aircraft. Trying to avoid that issue with very large diameter tail-mounted engines would just create a different set of design problems.
Comment: Not all; the [Pratt & Whitney Canada PW600](https://en.wikipedia.org/wiki/Pratt_&_Whitney_Canada_PW600) is a turbofan (albeit a _tiny_ one) with a centrifugal core compressor.
Comment: The RR Dart powers a propeller. Talk about a large diameter fan...


When is the earliest time you can daven ma'ariv during the week?

Question: When is the earliest time you can Daven Maariv on a weekday?
Comment: Gershon, can you be more specific?  Are you asking according to any shitta (in theory)?  Are you pressed for time?  Are you worried about contradicting the time for mincha?
Comment: Lchatchila and Bdieved
Answer: If you follow the "plag distinction", then Mincha can be davened until 1.25 relative-daylight hours before sunset, and Maariv any time after that.
If you follow the "sunset distinction", then Mincha can be davened until sunset, and Maariv any time after that.  I believe there are those who will allow Mincha until 11.5 or 13.5 minutes post-sunset, if you're really in a pinch, too.
The Gemara says either distinction is okay; some say that means a shul can do one way half the year and another way half the year; some read it that on any given day, you can do either way.  The most extreme version is to do Mincha after "plag" but Maariv before sunset; some shuls do this (Magen Avraham says it's okay for a community but not an individual), Aruch HaShulchan staunchly defends it, but it doesn't appear to be well supported by the sources (at least for an individual).  There are other ways to stretch it, e.g. using Magen Avraham's calculation of relative hours.
Now the evening recitation of Shema must be said after "tzeis hakochavim", when the stars come out.  Depending on your latitude and which rabbi you follow, let's call this very roughly 45 minutes after sunset (so long as you're no further from the equator than New York City).  (Yes I know someone is going to vote me down for not requiring 72, "Rabeinu Tam.")  The definition of "tzeis" is a very very complicated topic, so I'm oversimplifying here.
You can simply daven maariv right after sunset and then say Shema again later; ideally you could wait a bit so that the Shema of maariv would fulfill the evening-Shema mitzva.  If you don't daven maariv until 45 after sunset you're all set for the night; if you do it in between (let's say 18 after sunset) you should still repeat Shema, but it has some advantage as you fulfilled your obligation according to some opinions.
Comment: Who holds the distinction of the Rabbanan is sunset and not Tzeit? Also this post appears to conflate "Ma'ariv" and "Arvit" (that is, the blessings on the evening Shema and the evening Amida).
Comment: As I learned it, this is an instance where you need not be consistent from day to day (choosing which opinion to follow). But you should be consistent within one day.
Comment: @YehudaW That's not how the Shulchan Arukh and earlier authorities present it.


Unregistered Users Should Not Be Able to Suggest Edits

Question: It bugs me when unregistered users edit my Answers. It's not that big a deal to undo their edits (Moderators always seem to approve them almost immediately), but when they make really bad edits (and they always do) I can't communicate with them why their edts are lame.
Comment: (-1) Some anonymous edits are actually worthwhile. Though I do understand that some edits are annoying, or even invalid.  I'm very careful when approving edits, and I'm pretty sure that Mods are as well.  Also, the community requires at least two people to agree to an edit approval before it goes through. (the only exception is moderators where their votes are binding)
Comment: If bad edits are being approved on your posts, then maybe the review system needs fixing, but banning unregistered users from suggesting edits is not the solution.
Comment: How about presenting some examples to support your case? Right now we really have nothing to discuss about.
Comment: I doubt it's Moderators approving bad edits. You only need 2000 reputation points to participate in the Suggested Edits review queue. Also, this has been brought up multiple times on [metase] and it is quite emphatically `status-declined`.
Comment: @slhck I found three relevant edits: http://superuser.com/review/suggested-edits/44220 http://superuser.com/review/suggested-edits/52540 http://superuser.com/review/suggested-edits/52560 (not sure how much here is public) -- One of them only fixes a formatting issue with the previous edit.
Comment: Isaac: The link-formatting ones actually looks like an interesting addition, if only marginally relevant, other than the relevant fix of an error in your post you decided to keep. The other about processes I don't understand either. Please note that of five reviewers, only one was a moderator, and another user first approved of the edit as well. It seems you're just looking at what is effectively two edits, so *they make really bad edits (and they always do)* seems exaggerated. Feel free to provide more examples of bad anonymous edits for your argument.
Comment: I don't actually feel comfortable with approving added texts, because I always worry about the original intentions of the Poster. So if I had seen the earlier edit on the link-formatting post, I would definitely skip it (I approved the "HTML entity" one). But I guess not everybody skip edits when in doubt.
Answer: I agree that not being able to communicate with anonymous users is an issue, because there's no way you can tell them what is/is not wrong.
The following edit suggestion is a good example:
https://superuser.com/review/suggested-edits/53965

I should be adding another solution instead of editing someone else's
  solution, but I didn't know how to do that.

The suggestion is incorrect, because it's an "attempt to reply to or comment on the existing post", and I have no way of telling the person what to do instead.
What I'm suggesting is:

Require more attention from the reviewer when handling suggestions
from anonymous users.
Inform the anonymous users that they should register when making substantial changes/additions.
Or, limit anonymous users' capabilities for making substantial changes/additions.


Asus netbook trackpad suddenly stopped scrolling

Question: I bought an Asus Eee netbook three days ago and, up until today, my two-finger scrolling was working just fine.  All of a sudden, dragging two fingers in a downward motion does not scroll anymore.  I checked the Pointing Devices control panel icon and no matter what I change in the properties, I can't get it to start again.  Any ideas?
Comment: Do other multi-touch features like pinch zoom work? Have you cleaned the touchpad?
Answer: Start the Device Manager and look if there's a yellow icon somewhere. If there is one, try to look, if it is the driver you're searching for (Synaptics or smth like that) and install it.
Or it may be, that the driver is damaged, what means that you would have to search for the driver (in the device manager) and uninstall it, so that you can install it again (you probably will have to download it from the internet, just search for it)
For the Device Manager right click on Computer>Properties>Advanced System Settings>Hardware>Device Manager
On WinXP:
Right Click on computer >Properties>Hardware>Device Manager
Answer: You could try reinstalling the Synaptics driver - possibly installing the latest version. There has been an updated version available for mine on the asus site since I bought my eee.
If it is still not working then maybe warranty?


What were the optimal JPEG settings for Facebook photos in 2010/2011?

Question: Every time I upload a photo to facebook I'm disappointed; the photos just look really bad. What size do you recommend, dpi, etc? If it helps, I'm using lightroom.
Comment: Just to note, the resolution that Facebook uses has changed since July 2010. Many of the answers are now incorrect.
Comment: The there's a newer question with updated information. Normally, it's better to refresh the old question, but this one has so many answers that would need to be updated that I'm in favor of just marking this a duplicate of [What are the optimal JPEG settings for high-resolution Facebook photos?](http://photo.stackexchange.com/questions/27709/what-are-the-optimal-jpeg-settings-for-high-resolution-facebook-photos)
Comment: Good question, though I fear they may be nothing you can do - it may depend on Facebook's aggressive JPEG compression settings.

Post some examples!
Comment: **For a newer answer, see [What are the optimal JPEG settings for high-resolution Facebook photos?](http://photo.stackexchange.com/q/27709/1943)**
Answer: Facebook apparently applies a low pass filter (slight blur) to your images to make sure they compress better (or at least they used to), and then recompresses them at a higher ratio. The reason for blurring is that for a given jpeg quality (quantization) setting the more fine details you have the higher the filesize ends up being as these high frequency components can't be removed by the jpeg compression algorithm.
I wrote a custom action to prep images for facebook which identifies the out of focus areas, and blurs these whilst increases sharpening of other areas. This works for any system which compresses images to a fixed final filesize, as bits saved from the blurred areas are put to use providing more detail to the important areas!
For photos with a shallow depth of field this dramatically improves the overall quality without increasing the filesize.
Comment: Is there someplace to download this action?
Comment: @Jon I'll dig it you when I get a chance and post it here, however it's horrifically slow and inefficient in it's current form (I tend to run it overnight on large image sets) and needs redoing...
Answer: I use 72 dpi - Resize to fit checked - 720 pixels - Long Edge - Don't Enlarge checked.
Sharpen for Screen - High.
Quality 100 - sRGB - JPEG.
Comment: Actually Facebook's photo size is 720 pixels on the long edge, not 700.
Comment: I think the most important setting is the "700px long edge" as it probably keeps them from resizing it too much.
Comment: These are Lightroom settings, yes? What is the equivalent "Sharpen for Screen" in Photoshop CS4?
Answer: Facebook's current maximum image viewing size is 720 pixels on the long edge (or both edges for square images.
I export at 100% JPEG quality, 72 DPI (though this is mostly irrelevant), and use Sharpen for Screen - Standard to sharpen them subtly after Lightroom resizes them.
Here is an example of the result that Facebook ends up with a sharp, high dynamic range photograph. Saved from Facebook and re-uploaded to here.
Answer: In this case, DPI is very likely irrelevant. DPI (dots per inch) is about how many pixels in each direction fit in a given physical distance, and that is mostly a function of the user's monitor.
If the picture is, say, 700 px wide, then it will always be 700 px wide regardless of whether the image claims 72 dpi or 300 dpi. A commonly cited number is that monitors have 72 dpi, but I'm not sure how accurate that is these days. Assuming for a second that it is accurate, the 700 px wide image would be 700 px / 72 dpi = about 9.72 inches wide, physically.
The DPI setting of the image might very well matter in desktop publishing software, word processors, and so on, that actually target their output primarily towards print, but I think it's pretty safe to say that web browsers ignore it.
Comment: Why the downvote? The question does include asking about "recommended" DPI values (among other things).
Comment: Regarding monitor DPI, it's definitely changed these days. My 19" monitor is 10 inches tall and I use it at a 1440 x 900 resolution. 900 pixels by 10 inches gives a dpi of 90. I've heard 72 dpi quoted before, and I think it's an old standard referring to CRT monitors.
Comment: I think Macs used 96 dpi for a long time, maybe even from the beginning. It gets even more interesting when you take into account things like web browsers with the now fairly common full-page zoom. Bottom line, *when targetting computer screens*, I think DPI is, at best, more of a distraction than anything else. Pixel size is much more important in that case.
Comment: A downvote seems... wrong. But without addressing the actual question and just narrowing in on one point, it's more a comment than an answer.
Answer: Facebook has now increased the "long-edge" to 960 and all photos uploaded at 720 are "stuck" at the smaller rez.
Upload at the highest res you can because its most likely Facebook will increase the preview size in the future.
Answer: You should find out how large the image ends up being, and make it that size. That way it won't be resized, which reduces the sharpness.
Try to find out the maximum file size allowed, and adjust the quality setting to get below that size. Otherwise just save at best quality and let them reduce it when uploaded.
The PPI setting (often called DPI) is irrelevant for images that is shown on the screen. They are always sized according to pixels, not inches.
Comment: I'm skeptical - I'd assume that Facebook runs all images through its processing scheme, even if they don't need to be resized.

Anyway, this would be easy to test - upload an image and then compare what comes out with what you uploaded.
Comment: @Reid: It's the resizing that reduces the sharpness. If it's processed but remains the same size, the result is a lot better.
Comment: @Guffa, not so facebook recompresses JPEGs at a lower quality setting, and even appears to blur them slightly first to remove noise, or excess detail which would hurt the compression.
Comment: @Matt Grum: The JPEG compression also has a blurring effect at lower qualities, which might be what you see.
Comment: @Guffa It's possible, however as you take a sharp image and increase the JPEG compression you more often see a more artifacty result (specifically at block boundaries) rather than just a softer result, which is why I suspect they use a low pass filter first.
Answer: I usually export at 1440px (longest side), with strong sharpening. This is a 2x oversampling of the 720px that will be visible on the Facebook page. I got the best results this way (better than uploading a sharpened 720px image).
Note that it is a recommended sharpening method, to sharpen an image at exactly twice the final size and then downsize it accordingly. That way you get a very detailed result. The downsampling in this case is done by Facebook itself.
Note that people will be able to download that higher-res 1440px image as it is made available by Facebook (don't know if this can be avoided), and perhaps you don't want to publish that higher resolution image...
Answer: Today -- 2-29-12 -- I was messing around with Facebook's new Timeline conversion, since they're making us go there anyway, and could not get decent image quality AT ALL. What I finally did that seems to have worked really well is created an image that was exactly THREE TIMES the 72 ppi resolution -- 216 ppi -- and I left it at exactly five inches wide. (I wasn't going to screw around with also trying to figure out the physical size aspect.) Anyway, that worked really well and my final main Timeline image looked FAR sharper than it had before. FB definitely does some tweaking to the images we upload; this approach seems to have worked very well. Good luck....
Answer: I am not a photographer but I have experience with images for the web, including uploading images to facebook. It is my experience that what facebook does is resize an image based on the ratio between its KB size and its dimension (similar to what @Guffa said). For example, if an image is 815x315 and under 60kb Facebook wont touch it (at least not that I could notice). But if you upload a 100 kb image with the same dimensions Facebook will run it thru its JPEG compression).
What I do is save it as 72dpi JPEG at 100% quality (optimize checked) then run it thru www.jpegmini.com. So I don't know if @Matt Grum will share his action, but what he is  essentially doing is actually the heart of the JPEGmini "trick".


How can you get the URL of a wall post in Facebook?

Question: When you use the notification system to go to a post ("X friend commented on your post"), you are taken to a page with just that post, not your entire wall. You can also get there if you receive an email notification and click on see the message, but, how can you get that URL for any given post, without the notification/email?
Answer: Hover over the timestamp for the post.  It is a link very similar to the way it works on Twitter.
Comment: Sadly, its another example of the trend of looking clean at the expense of intuitiveness, google does it, and the iphone has a lot of examples also (who would ever intuitive think to drag an app onto another app). The tech generation (of which I am a part) will play enough or search to find these things out, but the less tech savvy and older generations really suffer.
Comment: Wow that is the least intuitive way to ever do that... I didn't even knew it was a link I thought it was just a label! Plus hovering shows the timestamp... how are you suppose to figure that its a link to the post! Well good thing we got webapps :) Thanks @rchern
Comment: +1 Just a few days ago I used a screen capture because I didn't think this was possible!
Answer: Rebecca Chemoff is right..though you don't need to hover. Just click on the "date" of the post. For example, on every post, there is the date, like "9 November" -- that contains a link. Click the date, and voila, the url of the post appears.


Is it allowed to read namaz with a non mehram?

Question: Assalam alaikum. I'm a 17_ year old girl from India . I go to tuitions in the evening here timings is from 5-8. And hence I had to read the Magrib Namaz(prayer) from the tuitions. And along with me there is one more guy who reads his namaz. We pray in the same hall but at the different corners. My question is that is it allowed to read namaz along with a non mehram? And is there anything that me or him are going against Islam which we are not aware of? Please do reply. Allah hafiz
Answer: You can not perform prayer alone with a non-mehram because you can not be alone with a non-mahram in any condition https://sunnah.com/riyadussalihin:990

No man must not be alone with a woman except in the presence of her
(Mahram).
Answer: no in general if there no neccessity you can do it but in islam it is said tha a woman should pray ta home and it is more better to pray in a room ana best is to pray in a corner of a room w r not even allowed to see a non mehram for second time than how can we say prayer with him even if we do it in different corners i do not think so u can pray after reaching home allah knows that u r not doing it willingly


Quotes for work by a tradesman in the UK: rules for VAT?

Question: If I receive a quote for work from a tradesman, and this quote is supplied as a document (PDF) which does not mention VAT as being either includes or excluded, can the tradesman later say that it didn't include VAT and demand I pay 20% more than the amount stated on the quote?
Comment: Did the tradesman give any indication that his business was VAT-registered? He can't charge VAT if he's not registered. Plus I was under the impression that quotes to "end users" (ie, not businesses) in the UK had to include VAT, but unfortunately I have no source for that.
Comment: To avoid any misunderstandings, escalation & uncomfortable situations without reason etc, I would clarify this beforehand without agreeing to anything. Thats far more reasonable than trying to deduce what is implied by a random PDF offer.
Answer: No, a tradesperson is not acting withing the law if they add VAT in an invoice if it wasn't also clear in the quote.
To do so is a misleading omission under The Consumer Protection from Unfair Trading Regulations 2008.
The Pricing Practices Guide section 2, while having no mandatory force, is easy to read and explains to traders that:

All price indications you give to consumers, by whatever means, should include VAT.
Comment: Exception: A tradesman with revenue of less than £82,000 per year doesn't have to be VAT registered, and if not VAT registered, must NOT include VAT on the bill, and would be stupid to include it on any quotes. It's rarely the case for a tradesman having a helper, or in a job where he supplies materials, which increase the revenue. Your window cleaner or gardener is quite likely not VAT registered.
Answer: There are two things to consider here, whether you will have to pay VAT on top of the price quoted, and whether you will be able to reclaim the VAT yourself. From the tags, I'd guess the latter isn't an issue, but I'll cover it anyway.
If the tradesman is VAT registered (and his/her VAT registration number should be on the quote if he/she is) then he/she can quote with or without VAT, but should specify on the quote. If it is not specified that you should add VAT, it should be safe to assume that the quote is for the price you pay, however watch out for all quotes ex. VAT in the small print of the trademans standard terms and conditions.
The best option would be to clarify with the trademan. A simple "Are you VAT registered, if so could you confirm that the quote is the price inclusive of VAT?" e-mail should suffice.
If the tradesman is VAT not registered, then they cannot add VAT. If they didn't mention their VAT number on their quote (which they should do if they are VAT registered) then this reduces your risk, but it's probably better off to confirm with them.
On the last point, which is unlikely to be relevant to you - If you are not VAT registered then you cannot reclaim VAT even if it was included.
Answer: If a price is quoted with no mention of VAT, then it includes VAT. So your tradesman can't then add VAT on top.
Comment: Would you have a reference to support this?
Comment: I fear it's personal experience. A company I worked for lost a five-figure sum by quoting a price without specifying that it excluded VAT.


No Market on Honeycomb for NOOKcolor

Question: I put Honeycomb on my NOOKcolor and the app market is not on it. Does anyone know how to fix it?
Answer: The nook devs site provides links and goes over downloading Google apps to your Nook color running honeycomb. Thissite lists the available Google app downloads from Cyanogenmod, explaining that they cannot come pre-installed due to licensing restrictions. The links at the very bottom of the page (and the first link) should help you find the appropriate applications.
Answer: Sometimes custom ROMs come without the Google apps (Gmail, Market, Maps, etc.) pre-loaded, in those cases it's a separate package that you need to flash over the ROM.  You should check with the ROM developer where you should get the gapps from.

No Market on Honeycomb for NOOKcolor


Does sodium form complexes like transition metal ions?

Question: I realise that there is a similar question here Difference between sodium ion and a transition metal ion dissolving in water? and it seems to answer my question, however I was reading about how aluminion ions form complexes in water, and aluminium is not a transition metal. So how can aluminium form complexes like transition metal ions, but sodium does not?
Answer: Although less common than transition metal complexes, sodium does form complexes with some ligands, particularly oxygen based ligands.
Aqua complexes are formed in aqueous solution, the most common being $\ce{[Na(H2O)6]+}$.
Sodium forms many complexes with crown ethers, cryptands and other related ligands. For example, 15-crown-5:
Answer: A sodium ion has no outer electrons, and only has a charge of one. The complexes it forms are mostly defined by the structure of the ligand(s) and electrostatics.
Simply speaking, a pure aluminium 3+ ion would be too highly charged to be allowed in condensed matter. The hughe electrostatic field would just rip off electrons from any unsuspecting neighbour. Complexes of Al3+ ions are partially covalent, involving ligand electrons "donated" into the outer (formally unoccupied) orbitals, making directional bonds to the ligand or to the eg. oxygen atoms in alumina.
Complexes of transition metals work the same, only they have more outer orbitals, which also have some electrons, so there are even more possibilities to form complex complexes. :-)


How to prove the existence of the limit of Markov transition matrix?

Question: Does the limit of a Markov transition matrix $M$:
$$\lim_{n\to\infty}M^n$$
 always exist? And if yes, how to prove it?
Answer: The limit does not always exist. The simplest counterexample is $M=\begin{pmatrix}0&1\\1&0\end{pmatrix}$, where $M^{2n}=I$ and $M^{2n+1}=M$ for every natural number $n$. However, the limit exists if all entries of $M$ are positive. This is a consequence of the Perron-Frobenius Theorem, whose proof is no trivial matter.
Answer: The limit does not always exist. You can see it just by consider an example, say take $M$ to be the $2\times 2$ matrix with $0$ on the main diagonal and $1$ off the diagonal. In the context of Markov chains it is instructive to understand why you can't expect convergence always. When convergence occurs, it means there is a stationary distribution, and thus that the stochastic behaviour in the long run becomes stable. So now, imagine there are two states and the probability of passing from one state to the other is $1$. Clearly, no stability can ever be achieved. Intuitively, this is because the system is not connected enough or does not mix states well enough, in some sense. This can be made precise, as if all entries of the Markov transition matrix are positive, then it can be shown that the limit does indeed exist.
Answer: If all entries are positive, the limit does exist and the rows of the limiting matrix are identical. One way to prove this is to show the difference between the maximum and minimum value of the column entries goes to 0 exponentially as the matrix is raised to higher powers. This is done, I believe, in Kemeny's book on Markov chains


Can a bad karma happen just only with a thought without any physical or verbal action?

Question: Do we collect bad karma just only with an instant thought without any physical action or verbal action?
Answer: Yes, from multiple suttas we see that karma can be done by way of the mind and intentional thoughts only. In fact, according to the first verses of the Dhammapada, the mind precedes all mental states. It's more important than words and actions.
From Nibbedhika Sutta:

"Intention, I tell you, is kamma. Intending, one does kamma by way of
body, speech, & intellect.

From Mahakammavibhanga Sutta:

After doing an intentional deed to be experienced as pleasant by way
of body, speech, or mind, one feels pleasure. After doing an
intentional deed to be experienced as painful by way of body, speech,
or mind, one feels pain. After doing an intentional deed to be
experienced as neutral by way of body, speech, or mind, one feels
neutral."

From Bhumija Sutta:

"When there is a body, pleasure & pain arise internally with bodily
intention as the cause; or when there is speech, pleasure & pain arise
internally with verbal intention as the cause; or when there is
intellect, pleasure & pain arise internally with intellectual
intention as the cause.

From Kamma Sutta:

And what is new kamma? Whatever kamma one does now with the body, with
speech, or with the intellect. This is called new kamma."
Comment: Great answer! I was heading down the Dhammapada route myself. ;-)
Comment: Great answer Ruben.
Answer: Just only with an instant unwholesome thought (without any physical action or verbal action) is generally not bad kamma; particularly if the true nature of that unwholesome thought is fully understood; but habitual unwholesome thoughts (without any physical action or verbal action) is certainly bad kamma because habitual bad thoughts are signs of inherent ignorance that block the progress of the Path.
Answer: The text below is taken from the collected teaching of Ajahn Chah - ' about being careful '
As It reads bad karma doesn't happen by sudden arising of thought.

So, some people feel that these sudden arisings of thought are wrong
or evil. You may have an impulse to kill someone. But you are aware of
it in the next instant, you realize that killing is wrong, so you stop
and refrain. Is there harm in this? What do you think? Or if you have
a thought about stealing something and that is followed by a stronger
recollection that to do so is wrong, and so you refrain from acting on
it – is that bad kamma It's not that every time you have an impulse
you instantly accumulate bad kamma. Otherwise, how could there be any
way to liberation? Impulses are merely impulses. Thoughts are merely
thoughts. In the first instance, you haven't created anything yet. In
the second instance, if you act on it with body, speech or mind, then
you are creating something. Avijjā has taken control. If you have
the impulse to steal and then you are aware of yourself and aware that
this would be wrong, this is wisdom, and there is vijjā instead. The
mental impulse is not consummated.
This is timely awareness, wisdom arising and informing our experience.
If there is the first mind-moment of wanting to steal something and
then we act on it, that is the dhamma of delusion; the actions of
body, speech and mind that follow the impulse will bring negative
results.
This is how it is. Merely having the thoughts is not negative kamma.
If we don't have any thoughts, how will wisdom develop? Some people
simply want to sit with a blank mind. That's wrong understanding.
Answer: I'm not sure -- there might be something about this in the Abhidhamma, which I don't know.
I think that when a thought arises, that's a result of past karma.
But an "action" can be mental, as well as (or instead of) physical or verbal. And for example, when a thought arises (as a result of past karma), then an action such as deciding whether that thought is skilful or unskilful -- that action is karmic.
Answer: Excellent question Tahlia.
Karma follows Dharma, of which intention is a part. You will find Karma is most present surrounding those who are of tranquil mind and heart.
To understand the nature of Karma in a nutshell:
Imagine a bubble that surrounds every individual. When an intention is created the shape of the bubble changes in order to support the intention we send out. If the intentions of the individual continue the bubble will become shaped by the intentions that define the individual.
The if the bubble is perfect it would be round, and a representation of the purest epitome the heart, body and mind of the individual could be, and 'slippery' (for lack of more accurate language) as far as Karmic bubbles go.
The chances of being afflicted by impure intentions of another are dramatically dropped when one is tranquil. When the impure intention slips off, the energy to muster that intention results in what is commonly referred to as 'bad' karma.
So it could be said, karma has little to do with the verbal or the physical. Intention is the wavelength of karma, verbal and physical responses are an easy way to observe karma in action. Lastly, if one does not let the intentions of another reach their inner world, they do not take part in the suffering.
Cultivate in harmony


Why is there an asteroid scene at the beginning and at the end of Birdman?

Question: Why is there is an asteroid scene at the beginning and at the end of the movie Birdman?
Did that asteroid strike Riggan and give him the power to fly?
Answer: I like to think this is a metaphor about Riggan's fall. Riggan is a part of a powerful and massive planet, but he left it (Planet Birdman) and he does not know to what he belongs.
While he falls, he is struggling and tries to hang on to anything around (friends, family). The bright light generated by an Asteroid could be his show, or his lost career).
He does not care about disappearing (he even wants to disappear as Birdman).
So for me it's just a movie summary, and it reminds us that an asteroid is not designed to fall, but to fly, until he meets an obstacle. And he is not crashing, which means to me: He is not falling, he is convinced he is, but he eventually will fly through space, without any bond.


Windows 7 system drive says it is raw, but System Recovery starts without issues

Question: I have been running Windows 7 RC1 since it was available a couple of months ago and had no issues whatsoever until today. When I start my laptop, Windows does not boot but instead Windows System Recovery starts. I've used diskpart to list the partitions on the drive and my system partition (c:) has a RAW filesystem. I really need to save all data on the disk as fast as I cant and I would really like not to have to reinstall my system.
Comment: you're not using an SSD as boot disk are you?
Comment: no, a standard SATA drive
Answer: Assuming its not a disk failure of some flavour (in which case you're out of luck), i'd suggest making an image of the disk with dd, or some rescue oriented version of it before you do anything else, just in case.
With that out of the way this sounds like the sort of thing testdisk excels at - run an analysis from it, and if your old partition is detected, you should be able to restore your partition.
Answer: I've been trough this trouble too, when i tried installing Hackintosh together with Windows 7 RC1.
There COULD be a new partition called 'System Recovery' or so...?
Check that, there should be some boot files, put them into the Windows 7 directory.
You can do that with stuff like the CMD from the Win7 Disk.
Now try to boot, you should come up into the Windows Boot Manager. It will probably tell you something about the boot isnt valid.
Now insert your Windows 7 Disk, Go to system repair, and system boot repair or so. It MIGHT happen that when you need to select the OS, there is nothing. Just continue.
This should work, however try to be carefull. I personnaly would recommanded a clean install on a new partition and then copying files etc but like you said you did not want that...  --> then just get a Windows 7 original copy, no RC.


What's the best MTU setting for a web server?

Question: I manage several web servers and their MTU settings were set at 1500 by default. Some of the machines have an MTU setting of 576. I've read a bunch about MTU and I feel I understand it well enough but I don't have a good sense for the current state of hardware between my router and my user's PCs.  Is 1500 a sane MTU to run on a public web server or will I be causing problems for some users?
thanks!
fz
Answer: The short answer: "It depends".
The longer answer:
normally the TCP implements path MTU discovery algorithm (RFC1191).
However, the problem is that the middleboxes sometimes drop the ICMP messages "Fragmentation Needed, DF bit set" that are used for this. That said, it is usually infrequent - so you should be ok with the default MTU of 1500 - assuming your side does not filter ICMP unnecessarily, the failures to connect will be caused by the misconfigurations on the client side.
the MTU of 576 is assumed if the host does not implement the Path MTU discovery (if you explicitly disable it). This will cause lower performance for the connections, so better do not touch it.
So, normally 1500 should be a reasonable MTU to run in the vast majority of the cases - unless a very specific case where the firewall administrator on your side is overzealously blocking ICMP and the remote client is connecting via a path with a link inbetween that has a smaller MTU. But still, this case is best fixed by reconfiguring the firewall, so again 1500 should be fine.
Comment: If you were stuck behind such a firewall, you might want to try something like 1400.  That leaves room for various packet encapsulation techniques that will use part of the default ethernet 1500 (e.g., PPPOE, IPSEC, GRE).  But, it leaves it at a reasonable size for relatively high performance.
Answer: Sure, 1500 is a sane value.
The best value really requires you to understand a lot of the hardware and protocols you use and how they work. 1500 is a very reasonable starting place.
I haven't used it myself, but the SG TCP Optimizer has been recommend to help you investigate your system.
That all said, the answer "it depends" is very right, and if I was trying to optimize my web server's speed I would start with something I have better control over.
Comment: Agreed. I'd definitely start with something else.
Comment: "It depends" was more about the specific circumstances. Usually you should not need to change it. I've updated my 'longer' answer with more details on what and why.
Answer: frankzappa. This paper answers your question exactly, and it's a quick read:
ftp://ftp.cup.hp.com/dist/networking/briefs/WebMTU.ps
Comment: Still, you should explain here why, not just link to some other resource.


What does it mean to borrow from the future?

Question: I've heard this phrase used in economics. But it seems singularly inapt, the future is never here - how can you borrow something that doesn't exist?
Answer: Since you have asked only the "how" part, I'd explain here only the "how" part of the well-known phenomena.
Economics 101.
That future isn't known implies that, (in economics) future is uncertain.
Since future is uncertain no one knows if the expected future income will actually materialize  or not when the future is actually here. Thus the one lending money against assumption of an having payment in the future carries a risk of non-payment. To compensate that risk the borrower agrees to pay "interest" to the lender.
That is how one is able to borrow from the future: by promising to pay extra in the future.
Thus it is just inexact language, for money is borrowed from the present only (in exchange of a promise to pay future money to lender). The borrower is borrowing money from lender who could had put money to some other use
Comment: Ok, so it's just a phrase: You're not actually *borrowing* from the future. You're compensating for risk of non-payment. I think you can look at it in a different direction - that the borrower is 'borrowing' what could have been done by the lender of that money.
Comment: Yes it is just inexact language, for money is borrowed from the present only (in exchange of a promise to pay future money to lender). And yes the borrower is borrowing money from lender who could had put money to some other use.
Answer: On one hand it is a weakness of language.
You are basically taking from your future potential by spending it today usually at a premium cost. All things being equal, if you invested your resources instead of your future earning potential (via debt/risks) for money today, you would likely have more resources instead of having the resources immediately with additional risks attached to it. Therefore, loosely you are borrowing money today at the cost of even more money tomorrow.
On the other hand I disagree with the future not existing. A more accurate statement based on physics is that you haven't experienced it yet. Just because you have not yet experienced it does not mean it is not real since unless your immortal you didn't experience most of the universes past either.
@Madhur I agree, this is more of an economics question.


How are anti-matter atoms created?

Question: What is the reaction, or reactions that make anti-matter? I don't understand how anti-matter is created by CERN if interaction with normal matter causes annihilation.
Answer: Anti-matter is created from interactions. They are created as a way to conserve energy and other quantum numbers such as charge. One such interaction would be the $Z^0$ decay:
$Z^0 → ν_e + \bar{ν_e}$
In this case lepton number is conserved.
They do annihilate producing energy (usually photons, but they can produce other particle-antiparticle pairs) when they interact/collide with their own antiparticle. They are produced in really low amounts therefore the annihilation is not as destructive as one might think from reading pop science.
Comment: Good remark on the anihilation not being as destructive. You need both particle and antiparticle to get close enough to interact!
Answer: For over forty years accelerator technology has been giving antiproton and positron beams, the easiest to create anti particles.
Positrons are the simplest because once the energy of electrons is accelerated to the values over pair creation of e+e-, the the brehmstrahlung photons, gamma ray energies, will create electron positron pairs when interacting with the field of target atoms, and the positrons can be lead with magnetic fields into beams.
Antiprotons are harder to create because a proton antiproton pair needs energies over the 2 GeV necessary for the rest mass of the baryon antibaryon pair. Antiprotons are created by accelerating protons to high enough energies hitting a target and generating a plethora of particles through the strong interactions. Among them there will be proton antiproton pairs and the antiprotons will be separated from the rest and guided into a storage and then beam acceleration systems. To create antihydrogen the antiprotons should be low enough energy to be able to capture positrons and create atoms, as described in the link.

How are anti-matter atoms created?


Delay in Premier League Football match

Question: Can a Premier League match be delayed due to a team arriving late on the pitch due to some reasons within the team? If yes, for how long will the referee wait for the team to arrive?
Comment: See http://www.skysports.com/football/news/11667/10238417/manchester-united-to-face-punishment-for-late-arrival-at-tottenham
Comment: It happened last season
Comment: Manchester United arrived late to all their London games last season. So much so that games against Spurs and West Ham had delayed kick offs.
Answer: This situation is described in Premier League Handbook 2016/17; Premier League Rules - Clubs: Operations - Section L: Fixtures - Kick-Off:

L.29. Each Club participating in a League Match shall adhere to the kick-off time and the Home Club shall report any delay to the Board
  together with any explanation therefor.
L.30. Any Club which without good reason causes to be delayed either the kick-off of a League Match from the time fixed or the re-start
  after the half-time interval:

L.30.1. shall on the first such occasion pay a fixed penalty of £5,000 if the delay does not exceed 15 minutes; and
L.30.2. shall on a second or subsequent occasion within two years of the first such occasion or if in any case the delay exceeds 15
  minutes be dealt with under the provisions of Section W of these Rules
  (Disciplinary section, which currently has a long list of 82 rules).
Comment: The Premier League Board (and all other football competition administrators around the world for that matter) aren't a court - they aren't bound by precedents and objective tests, unless such a test is defined in their regulations. If they decide on a specific occasion that traffic delays constitute "good reason" in their collective opinion, there is no sanction. They're not then bound to consider them "good reason" on a subsequent occasion, or for another club with similar circumstances (although it is probably in their best interests to do so).
Comment: I'm not sure this quite gets to the crux of the question due to the "without good reason" part of the rule; one of the most common reason for PL delays is traffic problems, which I imagine come under "good reason".
Comment: I'm sure in the context of an EPL game that has an average audience of 1 million TV viewers and an average attendance of 36 thousand spectators, traffic problems don't come under "good reason" (Unless you meant traffic accident of the club bus)
Comment: None of the Saturday three o'clock kickoffs are televised live (legally mandated), and if there are traffic problems, it's likely the spectators are delayed as well. The decision in that case will be generally be made with police advice, so I'd very much say that counts as a "good reason" unless you can provide evidence otherwise.

Delay in Premier League Football match


What are my odds of drawing the rest of my deck

Question: Do to the deterministic nature of the game, if I have a decent memory I can easily know what cards are left in my deck, if not their order. I have two cards in my hand that let me draw a card (1 each) and I know there are 4 cards left in my deck, two of which will also let me draw a card. What are the chances I'll be able to draw my entire remaining deck?
Comment: This is a fairly simple probability problem... if I knew the rules of Star Realms. What are the card drawing rules?
Comment: Some cards will allow you to draw a card under certain conditions, for this question, you can take it as read that the condition is filled. Simply, you an draw 2 of the 4 cards remaining, and if any of the drawn cards let you draw a card you can continue to do so.
Answer: 50%
Symbolizing the cards that give you a draw a D, and the cards that do not as N, there are 6 possible permutations of draw vs no-draw.
In each permutation, the second N card represents the farthest you can get through your deck on this turn.  I have bolded it for easier visibility.
DDNN
DNDN
DNND
NDDN
NDND
NNDD
Comment: This could also be explained by defining any permutation that ends with a Draw card last as the fail condition.


How do I reference another question and is it OK to post a question as a comment/answer on another question that relates to that question.

Question: A quick meta question, if I have a question that relates to another question, as this one does, do I post it as a comment? If not, how do I reference this question in my question? Thanks.
Comment: Perhaps related: [Clarify an old answer](http://meta.math.stackexchange.com/questions/4108/clarify-an-old-answer) (and maybe other posts [linked there](http://meta.math.stackexchange.com/questions/linked/4108)).
Answer: If you have a question that is related to another question, then post your question as a new question. You can then link to the related question and possibly say why they are related. You can simple paste the url of the related question into your question. It will automatically be formatted. For example
https://math.stackexchange.com/questions/140681/where-to-begin-with-foundations-of-mathematics
gives
Where to begin with foundations of mathematics
If your question is very similar, you should explain how your question is different (otherwise it might be closed as a duplicate.
You should never ask a new question in the comments.
Comment: I just figured that out in my last question I posted. Thanks. Plus the link you chose is a great one. Coming from a logic background, Linguistics and CS, it's doubly interesting to me. I'll start with Herbert Enderton's books A mathematical introduction to logic and Elements of set theory. I appreciate both answers.
Answer: Comments can be posted on Questions (and Answers) seeking a clarification or pointing out apparent typos, etc.  These should be "shallow" questions, in the sense that you expect the original author of the post would be able to address gaps in their post (or make a quick reply comment) that resolves the issue. [Often once the post itself is suitably edited, e.g. to cover a missing case, the Comments below it may be self-deleted by their respective contributors.]
More substantive problems should be posted as new Questions, with a link to the original helping to supply context.  As a rule of thumb I would ask myself whether the question being raised makes sense mathematically as a problem in itself, however much overlapping there might be, or if the question is really dependent on the formatting and phrasing of the earlier post (so not of independent mathematical interest).


Is there a master list of the Big-O notation for everything?

Question: Is there a master list of the Big-O notation for everything? Data structures, algorithms, operations performed on each, average-case, worst-case, etc.
Answer: Dictionary of Algorithms and Data Structures is a fairly comprehensive list, and includes complexity (Big-O) in the algorithms' descriptions.  If you need more information, it'll be in one of the linked references, and there's always Wikipedia as a fallback.
Comment: I downloaded all the pages and made a list off every page that provides big o: https://pastebin.com/X3c7i4aF
Answer: The Cormen book is more about teaching you how to prove what Big-O would be for a given algorithm, rather than rote memorization of algorithm to its Big-O performance. The former is far more valuable than the latter, and requires an investment on your part.
Answer: Try "Introduction to Algorithms" by Cormen, Leisersen, and Rivest.  If its not in there its probably not worth knowing.
Answer: In c++ the STL standards is defined by the Big-O characteristics of the algorithms as well as the space requirements. That way you could switch between competing implementations of STL and still know that your program had the same-ish runtime characteristics.
Particularily good STL implementations could even special case lists of particular types to be better than the standard-requirements.
It made it easy to pick the correct iterator or list type for a particular problem, because you could easily weigh between space consumption and speed.
Ofcourse Big-O is only a guide line as all constants are removed. If an algorithm runs in k*O(n), it would be classified as O(n), but if k is sufficiently high it could be worse than O(n^2) for some values of n and m.
Answer: Introduction to Algorithms, Second Edition, aka CLRS (Cormen, Leiserson, Rivest, Stein), is the closest thing I can think of.
If that fails, then try The Art of Computer Programming, by Knuth.  If it's not in those, you probably need to do some real research.
Answer: To anyone, who is coming to this question from Google.
http://bigocheatsheet.com/


Processing time for German job seeker visa

Question: I am applying for German job seeker visa at New York center on 1st week of December and I am planning to travel overseas towards the end of December. I need help to know what is the processing time for job seeker visa and how much time they will take for stamping.
Answer: From the Visa : Frequently Asked Questions - Federal Foreign Office the general answer is:

How long will I have to wait for my visa?
  The processing time for visa applications (short stay) takes up to 15 days.
If long-term residence permits or visa for employment in Germany are applied for, the processing time will be up to several months.

As a general rule, visas where consultation with the Aliens' Authority (Ausländerbehörde) are needed take longer.
When this is meantioned at the Visa for Germany or Employment in Germany

as it is for the student visa in the first site
or Jobseeker (the pdf form you will need to fill out) in the second

assume the time needed will be longer (1 to 3 months).


Key Things to look for in a Data Center

Question: I'm trying to build a simple checklist to determine the quality of a datacenter... where and what should I look for and how can I determine if what the owners say (e.g. "our UPS keep the data center up for 100 days without power") is true or not? What are typical signs or good or bad data centers?
Answer: Here is a list of questions I made for myself last time I went datacenter shopping:

Explain what it would take for sprinklers to go off on our equipment.
What will remote hands be willing to do?  For example, install hard drives, rotate tapes…
Are your remote hands available 24/7/365, average wait time for them to get to the cage after filing a ticket (How are tickets entered?) ?
Are you on multiple grids?
Do you have raised floor cooling?
How many datacenters do you operate besides this one?
How long can the datacenter run on backup power?
Can we have equipment delivered directly to the datacenter?
Is there a delivery dock and free, close, and available parking?
If we have a vendor come to the datacenter, do we need to accompany them?
What ambient temperature and humidity is maintained?
How many ISP choices are there?
Have any of your customers ever lost power for any amount of time in the history of the datacenter?
How long has this datacenter been in operation?
What access controls are in place to to both the floor and equipment?

If you visit several and ask these questions between the price, your visit impressions, and their answers it will probably be clear which one you want. Make sure you always visit them and visit a good amount of them.
Comment: I would also ask about access controls to both the floor and your equipment.
Comment: @JakeRobinson The new DC I moved into in 2003 had both FM200 and sprinklers. The fire marshals were very firm that there HAD TO BE sprinklers there. If the FM200 can quench the fire, great. But if it doesn't (UPS blow-out breaches the FM200 air-dam so O2 can get in after a dump) a lot of water will save the building. The right dry-pipe system means that if a sprinkler head gets bumped the DC doesn't flood.
Comment: Well done.  +1 to you.
Comment: Great list!! I would add: "Do you own the building?" Also, if they have sprinklers, walk away!
Comment: @JakeRobinson Sprinklers are required *by fire code*, even in datacenters, in many places. When you see sprinklers ask if they're *dry pipe* sprinklers.
Comment: @sysadmin1138 Fire suppression is fire code, but it doesn't have to be water. We use FM200 gas suppression in our datacenters. Dry pipe != dry chemical. Dry pipe just means the water is held at a valve.
Comment: Sometimes you can put "roofs" on your racks to reduce the risk of Sprinklers donig lots of damage, I also read somewhere about having drain whole in the bottom of all cases leading to much lower levels of damage
Comment: Also ask if how large the generators fual tanks are and if have proved they can refil them without mains power.   Power may be of for weeks if a few large city cables fail!
Comment: One more thing to ask, how often do they test. You can have as many generators, ups, redundant routers, etc, but they are all worthless if they are never tested.
Answer: Kyle covered it pretty well, but here are a couple points:
Physical Security is huge.  It should take nearly an act of Congress (Parliament, insert slow-moving bureaucratic institution here) to get inside.
It should have Halon fire suppression, not sprinklers; Servers should not be damp.  (Local fire-suppression regulations may override...)
Find out what their preferred server vendors are.  Unless it's for a very specific reason (like running a Google-like datacenter), it should be name-brand servers.  (Dell, HP, IBM, Sun, Apple, etc.)  If they say "white-box" or a brand that you don't recognize, run.  Note that there are some reputable lower-tier server vendors that are reputable (System76, for example), but "custom-built" means that they're putting things together themselves.  Great for your home, but bad for your datacenter.  (This doesn't include buying a HP Proliant DL580 and installing things like the memory option kits or drive cages.)
What ownership options are available?  Buy through them?  Buy direct and drop-ship there?  Leasing?  VMs?
Comment: Sprinklers are actually *required by fire-code* in many places. I found that out in 2003 during a DC build I assisted with. It's only supposed to fire if the FM200 doesn't quench the heat source. It should be a *dry pipe* sprinkler system backing up the inert-gas.
Comment: Agree on the fire suppression, although for "Halon" read "halon / FM200 / Argonite / Inergen / other extinguishant gas here"
Answer: Excellent as always Kyle,
A couple of things I've learned from experience:

Ask if there are generators to backup the UPS's, if so have the generators been tested, how often?
What physical locks and checks do they have in place to prevent electricians from killing the power?
What liability/insurance coverage do they have?
How do they deal with situations when they don't meet their SLA?
How often have they not met an SLA?
How much power do they provide to each rack/cage/etc.?  (Will you be power constrained and need another rack/cage just for the extra power?)
Ask for References, in your industry would be good.

Funny stories that weren't funny at the time:

There was a fire in Vancouver in an underground electrical compartment, 4 blocks from my DC, the fire took out the power for a 10 block radius.  The UPS' kept the lights on until the Gen-set came online.  Gen-set stayed online for about an hour before over-heating.  UPS's were able to keep the lights on for another 30 minutes after the gen-set did a safety shut down.  Gen-set belonged to the building, IIRC the DC was able to blame them and washed their hands.
An electrician killed the power to a couple rows of racks at the DC because the panel some how fell and knocked all the breakers open.  I've also heard about an electrician at another DC going to work on a UPS, not putting it into bypass mode and taking down the whole DC.
Comment: @AX1: UPSes in a daisy chain mess with each other. They expect certain characteristics in the in the current, and other UPSes can filter those out. The network will probably go down once the DC loses power, so even if your servers are up, the network won't be.
Comment: Is it advisable to have a rack UPS for those cases or does it not matter as the connection is usually taken down with everything anyways?
Comment: It would have been nice...  Our DC said we weren't allowed to have a UPS in our racks, they gave us a reason, but I don't recall what it was.
Comment: Maybe heat? I don't have much experience with rack UPS, but consumer UPS always heat up.
Comment: Could have been fire code, if the EPO is hit the firefighters expect there to be no live power.
Comment: @quinnr: Yes, it was the UPS's filtering the power.
Answer:
I would say that probably a third of a data center is the technical line items (Do you have {VESDA, re-fueling contracts, chillers covered by UPSes, multiple power grids, diverse fiber entrances).
Another third is how they deal with it when things don't go right.  Do they swallow their pride, examine the failure and figure out what to do to make sure it doesn't happen again?  Or do they just keep doing what didn't work before?
And the other third is the personnel.  Are they smart, easy to work with, and don't turn over every month?

But even more importantly: Do they have the space you need?  One place we almost went into, within 2 years we were using more space than they had available.
Answer: I work in a small data center in Silicon Valley. I'm the sysadmin on the managed-server side of the business.
Bad signs:

Lack of redundant monitoring and alerting for power, temperature, humidity
Lack of monitoring for network devices, colos, servers and other equipment
Clutter and not using cable-ties or other cable management to keep clean, organized racks

Good signs:
- Onsite diesel generator with automatic failover
- Backup chillers and air handlers with automatic failover
- Plenty of bandwidth on major carrier backbones (AT&T, XO Comm)
- Redundant network providers
- Redundant core routers, firewalls, load balancers and switches
- Running memory check and hardware diagnostics before deploying servers
Name brand servers are fine, but if they're old and have been around the block a bunch of times, you'd better make sure they're passing hardware diagnostics before using them.
A good data center should provide its customers with a website where they can monitor their bandwidth consumption and uptime. They should also answer any questions. Ask them the make and model of their UPS. Ask them to see the current load on the UPS. With this information you can verify how long it can go without power.
But honestly, the UPS should not be your concern. A UPS only provides a brief uptime (30 minutes or so). A much better concern is if the DC has a backup generator. It's also worthwhile to ask which grid the DC is on. In terms of brownouts and blackouts, different priorities are assigned to different grids. Guess what? Hospitals and fire stations are high priority (power is never cut). If the Data Center is on the same grid, its guaranteed reliable power.
Ask them how much power available per rack. Where I work we provide each rack with 3x 25amp circuits. A typical 1u server consumes 1-3 amp.
Comment: @AX1, 40A at 110V is 4400W; at 230V it's 9200W. Unless it's high-density (Twin/Twin^2/Blade) or HPC/GPU, that's PLENTY.
Comment: The one that I'm tempted to go to has 40amp per rack. Is that too low? It's almost half of what you say (75amp).

Key Things to look for in a Data Center