How to make newtonian particles to not go through each other?
hard self collision: make particles occupy space
How to get color of pixel in coordinates 123, 456 in screen? My AppleScript app needs to get color of pixel in coordinates 123, 456 in screen. I know that electromagnetic waves induce electric currents in conductors and that's the basis for radio, wi-fi etc.  I also know that light is also an electromagnetic wave. So, can light induce a current in a conductor (like a metal wire? or a coil?). And, if the answer is yes, is the same visible for other high-frequency waves (X-rays, gamma)?  I heard about the photoelectric effect, but it seems related to the particle theory of light (photons transferring their energy to electrons).  So, do high-frequency electromagnetic waves generate electric currents? Is it possible to measure them? Is the skin effect relevant here? I am a girl in my early twenties traveling to USA from India and in my visa application I have provided the contact address for a relative in San Francisco, and their invitation letter. I would like to visit LA first. Will I face problems during immigration if I land in LA (with an immediate on wards ticket to SF)? I dont plan to get on the flight from LA to SFO.  Initially I was planning to fly to SFO from India and then fly to LA immediately after completing immigration at SFO airport. These wonderful icon overlays that give you useful information in the Windows Explorer are not showing up.   It's the most annoying with Dropbox (I already read ). I just tried reinstalling Dropbox - no help. I have this problem also with the  so I don't think that it is a dropbox problem specifically; and something is wrong in my OS. The OS is actually a pretty fresh clean-install...  OS: Win8 Pro 64-bit. To post an image in a question/answer an user requires at least 10 rep points (see ). To be able to upload an image in the chat, the rep requirements is instead 100 rep point (see ).  Is there any reason to that? I am asking because IF the rationale is to provide more protection inside the chat, the protection is easily circumvented by uploading the image via the question editor, and then copying the link to the chat. Basically, the more restrictive requirement fail to reach its goal - assuming that blocking uploads from new users was indeed the target.  I am not proposing to change that (it doesn't seem worthy the effort) but I was curios to know if this is just a refuse caused by successive changes to the system or if there is actually a different rationale to this that I am missing.  [note: I will remove the double Discussion/Support tag as soon as I can focus if the question has a staff answer - support or will prompt a discussion] How to tell if a travel location is "affordable"? Disclaimer: The locations are just examples to set up the question and all prices are made up for the sake of the example and question.  Background: Let's say I want to visit London or Busan, and my currency is USD. While comparing the two locations I notice their exchange rates. 1 USD is 0.60 GBP, while 1 USD is 1,200 KRW.   Question: Are there traveler's tips and/or tools when it comes to currency exchanges where I know how much I can get for my "dollar" in a certain travel location? To use the exchange rate above as an example, consider the following. A bottle of water in London costs 0.40 GBP while a bottle of water in Busan costs 2,000 KRW. How would I know the relative value of goods I can get for my "dollar"?  I found  that provides that information but I'm wondering if there are other travel tips or anything to answer my question.   Edit: I replaced Korea with Busan to give it a more precise location and to keep it on the same level as London. how to pass a value to a variable that in a other variable I am getting a "bad substitution" error while running this code. I want to print the values in h_1, h_2, h_3with a for loop.  #/!/bin/bash h_1=12 h_2=13 h_3=14 for ((i=1; i&lt;=2; i++))  do   echo "${h_$i}" done nodejs vs node on ubuntu 12.04 I am looking for a list of all English words that are their own antonyms. Off the top of my head, I can only think of "either", "fast", "to dust" and "to lease", but there must be dozens more. Can you provide a link to a comprehensive list, if such exists?  Also, what are such words most commonly called? I like the term "Janus word" I once heard, but is that widely understood?  Edited by popular demand to make it absolutely clear that this is not a community wiki poll. Is the series $\sum_{1}^{\infty}\frac{1}{p_{j}}$ where $p_{j}$ is the $j$th prime convergent? Does the series $\sum_{n=1 }^{\infty}1/p_{j} $ of  reciprocal primes converge?  Experimentally, it seems convergent. What is a StackOverflowError? What is a StackOverflowError, what causes it, and how should I deal with them? The  is the stuff of legend () and is . My question is two-fold, really: Is the Golden Ratio really a good tool used in modern design (defined here as 20th century on forward), and if so, how often is it used outside of the aforementioned known uses?  EDIT: To @e100's first comment, examples of modern use of the Golden Ratio would be a great thing to add here if there any documented or arguably conclusive uses of it. $ KC $ spaces imply $ US $ spaces , but vise versa is false. In the $ US $ space , each convergent sequence has unique limit.      In the $ KC $ space , every compact subset is closed.      It easy to show that $ KC $ spaces imply $ US $ spaces.   The example below show that $ US $ spaces do not imply $ KC $ spaces.      Let $ X = [ 0, 1 ] $ with the the usual topology and attach  a new point z whose neighborhood are open dense subsets of $ [ 0,1 ]$. Observe $ [ 0,1 ]$ is a compact non-closed  of $X$ and then $X$ is not a $KC$ space. However no sequence in $ [ 0,1 ]$ converge to $z$ and in particular  all convergent in $X$ have unique limits.   can you give me more details?   Why $X$ is not closed? Why no sequence in $ [ 0,1 ]$ converges to $z$ and in particular all convergent sequences in $X$ have unique limits ? I'm trying to fix the following TikZ figure, where the circle device in the middle should be embraced by two bended arrows pointing counter-clockwise.    I have drawn these arrows with an arc path, but the tips of the arrows are unexpectedly both pointing upward.  Any idea how to get these arrow tips to follow the direction of the arc?  Here is my code for the above figure:  \documentclass[12pt]{article}  \usepackage{tikz} \usetikzlibrary{arrows} \usetikzlibrary{calc}  \begin{document} \begin{tikzpicture}   \draw (0,0) circle (1cm);   \draw[fill=gray!20]  (23:0.45cm)     -- (23:1cm) arc[radius=1, start angle=23, end angle=-203] (157:1cm)     -- (157:0.45cm) arc[radius=0.45, start angle=157, end angle=23] (23:0.45cm);   \draw[fill] (0,0) circle(2pt);   \draw[-&gt;,&gt;=stealth',semithick] (150:1.2cm) arc[radius=1.2, start angle=150, end angle=210] (210:1.2cm);  \draw[-&gt;,&gt;=stealth',semithick] (-30:1.2cm) arc[radius=1.2, start angle=-30, end angle=30] (30:1.2cm);   \foreach \x/\y in {1/0,2/0,3/0,4/1,5/1,6/1,7/0,0/1}     \node[rotate={\x*45-90}] at ($(0,0)!.75!\x*45:(1,0)$) {\y} ; \end{tikzpicture} \end{document} Anamorphic Perspective in Illustrator or Photoshop? How can I efficiently recover a permanently deleted folder at once? What would be the value of $\sum\limits_{n=0}^\infty \frac{1}{an^2+bn+c}$ I would like to evaluate the sum  $$\sum_{n=0}^\infty \frac{1}{an^2+bn+c}$$  Here is my attempt:    Letting    $$f(z)=\frac{1}{az^2+bz+c}$$  The poles of $f(z)$ are located at   $$z_0 = \frac{-b+\sqrt{b^2-4ac}}{2a}$$  and  $$z_1 = \frac{-b-\sqrt{b^2-4ac}}{2a}$$  Then  $$ b_0=\operatorname*{Res}_{z=z_0}\,\pi \cot (\pi z)f(z)= \lim_{z \to z_0} \frac{(z-z_0)\pi\cot (\pi z)}{az^2+bz+c}= \lim_{z \to z_0} \frac{\pi\cot (\pi z)+(z_0-z)\pi^2\csc^2 (\pi z)}{2az+b} $$  Using L'Hopital's rule. Continuing, we have the limit is  $$ \lim_{z \to z_0} \frac{\pi\cot (\pi z)+(z_0-z)\pi^2\csc^2 (\pi z)}{2az+b}= \frac{\pi\cot (\pi z_0)}{2az_0+b} $$  For $z_0 \ne 0$  Similarly, we find  $$b_1=\operatorname*{Res}_{z=z_1}\,\pi \cot (\pi z)f(z)=\frac{\pi\cot (\pi z_1)}{2az_1+b}$$  Then   $$\sum_{n=-\infty}^\infty \frac{1}{an^2+bn+c} = -(b_0+b_1)=\\ -\pi\left( \frac{\cot (\pi z_0)}{2az_0+b} + \frac{\cot (\pi z_1)}{2az_1+b}\right)=  -\pi\left( \frac{\cot (\pi z_0)}{\sqrt{b^2-4ac}} + \frac{\cot (\pi z_1)}{-\sqrt{b^2-4ac}}\right)= \frac{-\pi(\cot (\pi z_0)-\cot (\pi z_1))}{\sqrt{b^2-4ac}}= \frac{\pi(\cot (\pi z_1)-\cot (\pi z_0))}{\sqrt{b^2-4ac}} $$  Then we have  $$\sum_{n=0}^\infty \frac{1}{an^2+bn+c} = \frac{\pi(\cot (\pi z_1)-\cot (\pi z_0))}{2\sqrt{b^2-4ac}}$$    Is this correct?  I feel like I made a mistake somewhere.  Could someone correct me?  Is there an easier way to evaluate this sum? Add links to official status pages in the read-only mode status message When the site is in read-only mode, I get a message like this:     This site is currently in read-only mode.   Would it be possible to update that message to provide links to these resources on SE's status:   : Official SE tweets on SE's status. : Official blog posts on SE's status. Good 1st PDE book for self study How to configure idle period for auto locking screen I just installed a fresh Ubuntu 12.04 (Precise Pangolin).   I noticed that, there is no screensaver settings, which is fine. I like to control the period of time after which the blank screen is shown,which automatically locks the screen.  I do not want to enable screensaver. I only wish to be able to change the idle time period required to lock the screen automatically. It seems to me, it is by default around 10-15 minutes.