How to tell if user goes offline
How to detect online/offline event cross-browser?
How fast does gravity propagate? $1,000 is pronounced as "one thousand dollars". Reading from left to right, it seems like it would make more sense to write the value as: 1,000$. This way the pronunciation of the value follows the way it's written. Every other numerical value I can think of places the unit after the value:   1 kg 1 L 1 m, etc...   I'm not sure if this question fits in with the rest of the questions on this site. But I'd like to know why currencies are so different from other quantities. Not to mention the need to separate groups of three digits with commas. Subgroups of finitely generated groups are not necessarily finitely generated I was wondering this today, and my algebra professor didn't know the answer.      Are subgroups of finitely generated groups also finitely generated?    I suppose it is necessarily true for finitely generated abelian groups, but is it true in general?  And if not, is there a simple example of a finitely generated group with a non-finitely generated subgroup?  NOTE:  This question has been merged with another question, asked by an undergraduate.  For an example not involving free groups, please see Andreas Caranti's answer, which was the accepted answer on the merged question. If an object is large enough and slow enough is there a point that we can say we know its position and momentum? How do I install an SSH private key generated by PuTTYgen? Control custom properties How can I add remove and create a callback/event for a custom property using bpy. I have a scene, called scene, and I want to add a custom property named x, defulat value 2 (as intereger), and a callback foo, to run when it is changed. I also need to be able to read the value from it, and delete it. How do I do this? How do I remove directories that begin with a dot after my server was infected? My server had been infected and the attacker spread a bunch of .5randomletters folders around my file system, with malicious code inside of them. I want to search for all the folders that begin with a dot, review them and then remove them.  How can I do that? How to know the motherboard BIOS version using the command line How can get the BIOS version of my motherboard?  The command dmidecode gives you some information about the BIOS, but I can't find what I need...    I tried dmidecode command before asking and I didn't find what I needed - I want only the BIOS version and nothing else in the output. I can't find .bash_profile in Ubuntu 14.04 in my /home/user directory. I used the ls -a command to see the .bash_profile, but there isn't such a file. Are vacuum fluctuations really happening all the time? In popular physics articles and even some physics classes I've been to, the vacuum of space is described as being constantly full of quantum fluctuations.  Supposedly, all sorts of particle-antiparticle pairs at all scales are constantly appearing and disappearing.  We end up with a mental image of the vacuum as a roiling, choppy sea with all sorts of things going on, rather than a calm, placid background.  However, the vacuum, being the lowest-energy state of a theory, should be an energy eigenstate—which means it is time-invariant (except for a physically-irrelevant phase factor).  So it seems the vacuum really should not be seen as a dynamic entity with all kinds of stuff happening in it, as we're led to believe.   that     A “vacuum fluctuation” is when the ground state of a system is measured in a basis that does not include the ground state; it’s merely a special case of a quantum fluctuation.   So it sounds as if the existence of vacuum fluctuations is contingent on measuring the vacuum—in particular, measuring something that doesn't commute with energy (such as, I guess, the value of a field at a point).  How much truth is there to the idea that vacuum fluctuations are constantly happening everywhere, all the time?  Is that really a useful way to think about it, or just a myth that has been propagated by popularizations of physics? Arabic Stack Exchange? I have been learning some Arabic mostly for fun. I am looking for the Arabic Stack Exchange site but cannot find it. I am sure I saw it lying around a few months ago or so. Any idea what happened to it? I am a novice user and I am trying to install Oracle Java. The process calls for me to access /usr/local/java, but here is the problem. I get as far as /usr/local, then this is the message I get:  leonard@leonard-MT6452:/usr/local$ dir bin  etc  games  include  java  lib  man  sbin  share  src  leonard@leonard-MT6452:/usr/local$ cd /java/ bash: cd: /java/: No such file or directory  leonard@leonard-MT6452:/usr/local$ rm -r ~/java rm: cannot remove ‘/home/leonard/java’: No such file or directory   Help? How can I fix Print Composer errors due to huge file size? I want to extract infromation from logs using a shell script for a particular time range . A line in logs looks like this:  [14:58:27:176][15-10-2015]: testing1-vidya-Thu Oct 15 11:49:27 IST 2015 [14:59:27:178][15-10-2015]: testing2-vidya-Thu Oct 15 11:49:27 IST 2015 [15:00:27:179][15-10-2015]: testing3-vidya-Thu Oct 15 11:49:27 IST 2015 [15:01:27:180][15-10-2015]: testing4-vidya-Thu Oct 15 11:49:27 IST 2015 [15:01:27:181][15-10-2015]: testing5-vidya-Thu Oct 15 11:49:27 IST 2015 [15:02:27:182][15-10-2015]: testing6-vidya-Thu Oct 15 11:49:27 IST 2015 [15:02:27:183][15-10-2015]: testing7-vidya-Thu Oct 15 11:49:27 IST 2015 [15:02:27:184][15-10-2015]: testing8-vidya-Thu Oct 15 11:49:27 IST 2015 [15:02:27:191][15-10-2015]: testing9-vidya-Thu Oct 15 11:49:27 IST 2015 [15:03:27:192][15-10-2015]: testing10-vidya-Thu Oct 15 11:49:27 IST 2015 [15:03:27:193][15-10-2015]: testing11-vidya-Thu Oct 15 11:49:27 IST 2015 [15:03:27:208][15-10-2015]: testing12-vidya-Thu Oct 15 11:49:27 IST 2015 [15:04:27:209][15-10-2015]: testing13-vidya-Thu Oct 15 11:49:27 IST 2015 [15:49:27:210][15-10-2015]: testing14-vidya-Thu Oct 15 11:49:27 IST 2015 [13:02:17:209][15-10-2015]: testing13-vidya-Thu Oct 15 11:49:27 IST 2015 [15:04:27:209][16-10-2015]: testing13-vidya-Fri Oct 16 11:49:27 IST 2015 [15:49:27:210][16-10-2015]: testing14-vidya-Fri Oct 16 11:49:27 IST 2015   I need to extract data in specific intervals. for example in between 14:59 to 15:03 on 15-10-2015 as shown below:   [14:59:27:178][15-10-2015]: testing2-vidya-Thu Oct 15 11:49:27 IST 2015 [15:00:27:179][15-10-2015]: testing3-vidya-Thu Oct 15 11:49:27 IST 2015 [15:01:27:180][15-10-2015]: testing4-vidya-Thu Oct 15 11:49:27 IST 2015 [15:01:27:181][15-10-2015]: testing5-vidya-Thu Oct 15 11:49:27 IST 2015 [15:02:27:182][15-10-2015]: testing6-vidya-Thu Oct 15 11:49:27 IST 2015 [15:02:27:183][15-10-2015]: testing7-vidya-Thu Oct 15 11:49:27 IST 2015 [15:02:27:184][15-10-2015]: testing8-vidya-Thu Oct 15 11:49:27 IST 2015 [15:02:27:191][15-10-2015]: testing9-vidya-Thu Oct 15 11:49:27 IST 2015 [15:03:27:192][15-10-2015]: testing10-vidya-Thu Oct 15 11:49:27 IST 2015 [15:03:27:193][15-10-2015]: testing11-vidya-Thu Oct 15 11:49:27 IST 2015 [15:03:27:208][15-10-2015]: testing12-vidya-Thu Oct 15 11:49:27 IST 2015   I am new to shell script.I have tried with grep command . Can anyone suggest me how to do this ? It's so annoying to drag it over and over again. How to change default position of a console? I know why questions, answers and some other aspects of the site are color-coded and what they mean but I can't find why badges are color-coded. I'll include a snapshot to show what I mean. Trouble trying to use Windows Search to search for file with a question mark in the content It seems that Windows Search is having trouble when I specify a question mark character as the content that I'm searching for within my files.  I've tried:   content:? content:"?" content:"\?" content:\? content:%3f content:"%3f" content:\3f content:"\3f" content:\00\00\00\3F   but each of these yield no results when I do indeed have files that contain ? within the scope of the search. If I instead search for something like content:happy it will indeed find all files with the phrase "happy" within those same files.  My assumption is that Windows Search is treating some characters such as the question mark as a special character likely for the purpose of wild card expressions. To test this theory I've also searched for files containing an asterisk * and indeed I have the same issue as when searching for content of question mark. What I'm wondering is whether there is any way to get a search to work where I can search for a literal expression without any type of wildcard matching or at least a way to escape the special characters within the expression.  By the way I'm using Windows 7 Enterprise. Real Time LaTeX? Possible Duplicate:          I'm looking for an application in which I can write pure LaTeX in one half of the screen and on the other half to see the compiled document in real time. This seems really like an obvious way to do LaTeX authoring but it doesn't look like any software is able to do it. LyX is not what I'm looking for. I want to edit the actual LaTeX and see it rendered in real time. Why isn't this already being done somewhere? Why does a force not do any work if it's perpendicular to the motion? I have a book that says the Moon's orbit is [in this context assumed to be] circular. The Earth does no work on the moon. The gravitational force is perpendicular to the motion. Why is there no work done if support force is perpendicular to the motion? I need to find all functions $f:\mathbb R \rightarrow \mathbb R$ which are continuous and satisfy $f(x+y)=f(x)+f(y)$ I need to find all functions $f:\mathbb R \rightarrow \mathbb R$ such that $f(x+y)=f(x)+f(y)$. I know that there are other questions that are asking the same thing, but I'm trying to figure this out by myself as best as possible. Here is how I started out:  Try out some cases:  $x=0:$ $$f(0+y)=f(0)+f(y) \iff f(y)=f(0)+f(y) \iff 0=f(0) $$ The same result is for when $y=0$  $x=-y:$ $$f(-y+y)=f(-y)+f(y) \iff f(0)=f(-y)+f(y) \iff 0=f(-y)+f(y)\iff \quad f(-y)=-f(y)$$ I want to extend the result of setting $x=-y$ to numbers other that $-1$, perhaps all real numbers or all rational numbers. I got a little help from reading other solutions on the next part:  Let $q=1+1+1+...+1$. Then  $$f(qx)=f((1+1+...+1)x)=f(x+x+...+x)=f(x)+f(x)+...+f(x)=qf(x)$$ I understood this part, but I don't understand why this helps me find all the functions that satisfy the requirement that $f(x+y)=f(x)+f(y)$, but here is how I went on:  Thus  $$f(qx)=qf(x)$$ and it should follow that $$f \bigg (\frac {1}{q} x\bigg)= \frac{1}{q}f(x)$$ where $q\not =0$, then it further follows that  $$f \bigg (\frac {p}{q} x\bigg)= \frac{p}{q}f(x)$$ where $\frac{p}{q}$ is rational, and lastly it further follows that  $$f (ax)= af(x)$$ where $a$ is real. Thus functions of the form $f(ax)$ where $a$ is real satisfies the requirement of  $f(x+y)=f(x)+f(y)$.  I don't know how much of what I did is correct\incorrect, and any help would be greatly appreciated. Also is there any way that I can say that functions of the form $f(ax)$ where $a$ is real are the only functions that satisfy the requirement of  $f(x+y)=f(x)+f(y)$? Or do other solutions exist?  Again, thanks a lot for any help! (Hints would be appreciated, I'll really try to understand the hints!)