Creating multiple variables
How do I create variable variables?
An excited electron loses energy in the form of radiation. The radiation constitutes photons which move at a speed $c$. But is the process of conversion of the energy of the electron into the kinetic energy of the photon instantaneous? Is there a a simple way to visualize this process rather than math? How to get Steam to recognize existing game files from a different source? Let's say I have a game bought on retail/other digital download and installed on my hard drive. I buy the same game (or a minor variant of it, say the GOTY edition) on Steam. It should be possible to use the existing installation to avoid the generally big downloads. This is very useful as it saves both bandwidth and time.   Previously, as far as I understood, Steam downloaded the game files directly to steam\steamapps\common\&lt;game_name&gt;. Since this folder would be created on starting the download, pausing and closing steam, copying the game files to this folder and then validating game files would make Steam absorb as much as possible and just download the missing files.  Currently, Steam downloads the files to steam\steamapps\downloading\&lt;game_id&gt; and then moves them over to the aforementioned folder once the download finishes. So first of all, what the final folder would be is not clear. So what would be the correct way of doing the same thing, that is, make steam use as much of the existing files as possible? Should the files be copied to downloading\*\, or should I figure out the actual folder (say via googling) and copy the files to steamapps\common\*\?  More specifically, I had a retail copy of Arkham City installed and I just bought the GOTY version on Steam. Now I tried both of the above things, in either case, the validating does not seem to do anything. Assuming I have the correct folder name steamapps\common\Batman Arkham City GOTY, according to , if I copy the files there, Steam seems to just continue with its 17GB download. If I copy it to steamapps\downloading\200260 then the download keeps stopping every few seconds with "Disk Write Error" after modifying 1 or 2 files, and I have to keep hitting resume. Shutdown Cause -128 How get the :hover css style of an anchor with jQuery? How can i get the :hover in css stylesheet on the fly with jquery?  stupid example:   a.foo {     color: red;     font-size: 11px; }  a.foo:hover {     color: blue;     font-size: 12px;  }   how to retrieve that color and font-size before that mouse will go over the anchor? How do you make a test class for custom controller? My search results involves this but trying to apply it in my test class only create too many error that I don't understand.  public static testMethod void testMyController() {     PageReference pageRef = Page.success;//Observe how we set the Page here     Test.setCurrentPage(pageRef);   . Does my test class need those line of codes when my controller class is only very simple.  public class ClientInfo_VFcontroller{     public Org_Info__c orgInfo {get; set;}     public ClientInfo_VFcontroller(){         orgInfo = new Org_Info__c();     }     public PageReference save(){         insert orgInfo;         orgInfo = null;         ScheduleRetrieve.ScheduleRetrieve();         return null;     } } Example of a finitely generated module with submodules that are not finitely generated I'm looking for an example of a finitely generated module with submodules that are not finitely generated.  I've found  dealing with group (i.e. an example of a finitely generated group with subgroups that are not finitely generated). But I can't figure out whether that question do help to this one.  And I actually want to find a more "module-like" example rather than an example derived from a 'strange' group.  Can you please help? Thank you! Is this algebraic identity obvious? $\sum_{i=1}^n \prod_{j\neq i} {\lambda_j\over \lambda_j-\lambda_i}=1$ If $\lambda_1,\dots,\lambda_n$ are distinct positive real numbers, then  $$\sum_{i=1}^n \prod_{j\neq i} {\lambda_j\over \lambda_j-\lambda_i}=1.$$ This identity follows from a probability calculation that you can find at the  top of page 311 in the 10th edition of Introduction to Probability Models by Sheldon Ross.   Is there a slick or obvious explanation for this identity?   This  question is sort of similar to ; clearly algebra is not my strong suit! Problem from Herstein on group theory The problem is:  If $G$ is a finite group with order not divisible by 3, and $(ab)^3=a^3b^3$ for all $a,b\in G$, then show that $G$ is abelian.  I have been trying this for a long time but not been able to make any progress. The only thing that I can think of is: $$ab\cdot ab\cdot ab=aaa\cdot bbb\implies(ba)^2=a^2b^2=aabb=(\text{TPT})abba.$$ Now, how can I prove the last equality? If I write $aabb=abb^{-1}abb$, then in order for the hypothesis to be correct, $b^{-1}abb=ba\implies ab^2=b^2a$. Where am I going wrong? What should I do? To flag or not to flag question that don't show own effort? I am a little confused about what do with questions that are just asking      "I want this and that (...) and I want it to look exactly like that   (...) please help"   without showning ANY own effort?  According to  those kind of answers are not welcome  also the  says those questions should not be asked:     Don't ask about...   Questions you haven't tried to find an answer for (show your work!)   But does this mean that I should flag those kind of "helpdesk" questions or not? And if yes as what? low quality ? flag for moderator attention ? Or is just adding a kind comment asking for the asker's own attempts enough? I am having an access problem to some files and directories that I have tracked down to the group assigned to access them.    When I manually change the name of the group in the properties/permissions menu for a directory from the default setting (MyGroup) to www-data used by my Drupal Website, the directory error messages I get are gone.    There are a large number of files and directories that require this change.  If believe using chmod will change the owner which is set correctly and being new to Ubuntu, I am reluctant to experiment without a better understanding of how owner and groups work in the permissions settings.   What command changes the Group setting for a directory? How to list only JPEG files from root below using the command line? Prove that $\phi(n) \geq \sqrt{n}/2$ I am having trouble getting a basic example of a matrix of notes to show up in my beamer presentation. I am using the code copied from the pgf manual. Here is a complete working example. The error I get upon compilation is:  "Package pgfbasematrix Error: Single ampersand used with wrong catcode."  \documentclass{beamer} \usepackage{tikz} \usetikzlibrary{arrows,decorations.pathmorphing,backgrounds,fit,petri,positioning,matrix}  \begin{document}  \begin{frame}  \begin{tikzpicture} \matrix (magic) [matrix of nodes] { 8 &amp; 1 &amp; 6 \\ 3 &amp; 5 &amp; 7 \\ 4 &amp; 9 &amp; 2 \\ }; \draw[thick,red,-&gt;] (magic-1-1) |- (magic-2-3); \end{tikzpicture}  \end{frame} \end{document} What's the difference between "speak" and "talk", grammatically speaking? There are a number of questions e.g.  and  that deal with the slightly different connotations of the words "speak" and "talk". However, there also seem to be some grammatical differences between the two words. This question is about whether there's a way to formally pin down these differences, or whether they're just contingent features of the two words having different histories.  Some examples of grammatical differences are as follows:   be talkative ... be speakative speak French ... talk French give a talk ... give a speech (as opposed to a speak) speak up ... talk up (the meanings of the two phrases being completely different, with "up" being a preposition only in the latter case) speaking of which ... talking of which grammatically speaking ... grammatically talking   On the other hand, many other constructions work just as well with either (though they might have subtly different meanings), for example   speak to ... talk to speak with ... talk with speak about ... talk about   I'm interested in whether there's a way to pin down these differences (e.g. are the two words classified as different types of verb in some way?), or whether they're essentially just arbitrary. I'm also interested in why we have these two different words with subtly different meanings. Is the distinction between “speak” and “talk” a feature of many languages, or is it just a peculiarity of English?  Etymologically, both words are from Germanic origins. “Talk” seems to have been formed from the Middle English tale, even though “speak” already existed in the English language by then (as far as I can tell). This makes it even more mysterious: Why did we form a new word as a synonym of one that was already established? Or were the meanings different at that time? Can spells be cast through a Wall of Force? Is there a printable version of a reference manual for all TeX primitives? How do I remove the proprietary ATI drivers? I've installed 11.10 and the proprietary ATI drivers using "additional drivers" The performance of my system is absolutely awful and it shouldn't be. I tried to remove the proprietary drivers using the Additional Drivers tool and it appears to remove them. However after I reboot I cant get back into my desktop properly (the panel and launcher go missing). This doesn't seem to be an isolated problem in 11.XX.  covers how to restore the desktop (panel and launcher), but the guide doesn't fix my problem though.   Whenever I do sudo unity --reset it runs through its normal processes until it hangs at setting update "run_key" and never gets past that. I must reinstall the proprietary drivers using jockey-text or jockey-gtk in order to get back to my proper desktop.   Interestingly enough the system performance seems improved while it is in its "broken" state (missing panel and launcher).  I think restoring the default drivers may solve my problems but I cant figure out how to do it. Why is the time limit for editing comments 5 minutes? What is the reasoning behind this? How to fix slow wireless on machines with Intel wireless cards? How to install MySQL workbench? Installing MySQL workbench on 10.04 or 10.10 is not a problem, as the deb packages are available on MySQL's website.  But there is no 11.04 deb package available.  So, how do I install MySQL workbench on 11.04 and keep it updated automatically when updates become available to ubuntu via apt-get update.