Any hint to prove   $$ \sqrt{x+y} \le \sqrt{x} + \sqrt{y}, \qquad \forall x,y \ge 0 $$
How do I prove $\sqrt{x + y} \le \sqrt{x} + \sqrt{y}$? for $x, y$ positive?  This should be easy, but I'm not seeing how.  A hint would be appreciated.
Integral Inequality $|f''(x)/f(x)|$ Let $f$ be a $C^2$ function in $[0,1]$ such that $f(0)=f(1)=0$ and $f(x)\neq 0\,\forall x\in(0,1).$ Prove that  $$\int_0^1 \left|\frac{f{''}(x)}{f(x)}\right|dx\ge4$$ Run a command in a shell and keep running the command when you close the session I am using Putty to connect to a remote server. What I want to know is if there is any way to write my commands and allow them to keep running after I close the session with Putty. The reason for this is that I do not want to keep the computer ON all the time. Is there any way to do this?.  Update with the solution  For my question as it is presented the best solution is use one of the commands provided such as nohup, because you do not have to install any additional software. But if you are in the same problem use screen, install it and use it. It is amazing.  I have selected the answer of Norman Ramsey as favourite because propose several solutions using commands and screen. But please check the other answers specially the one of PEZ, then you get an insight of what screen is able todo. The question is pretty straight forward. I can get the exit code of the previous command by using $?. But what exactly is it?  I don't think it is a variable, because you can't have any special characters in the name of variable other than _.  foo?foo=10   will result in the error: foo?foo=10: command not found  So if it is not a variable, what is it? Are there others like it? Stack Exchange iOS app nearing beta We're getting very close to releasing our iOS app on the App Store.  Since the , we have taken on hundreds of users, fixed a lot of bugs, and we have approached feature completion for v1.0.  We're still working on finishing touches and fixing crashes - but there aren't too many of those these days.  We believe we'll have this out in .    Current Alpha Testers:  We want to extend to you a huge thank you for using our app and giving us feedback.  We've made a ton of improvements that filtered through  here on meta.  We're pretty greedy, though, and we want more of your feedback as we get close to beta.  If you've been using the app, please leave an answer to tell us how you think we've done so far.  Don't go easy on us - we want to get this as right as we can before hitting the App Store.  You might even consider telling us how many stars you would give us if we were in the store.  Also, we created a room called  for more casual discussion about the app.  Everyone Else:  The alpha will continue in the interim, and we still .  The invitations are sent out once a week (usually), so if you don't get an invite right away, don't panic.  We'll get to you.  Bugs or feature requests should be reported here on meta and tagged .  Update, 5/1/2014    As of today, the app is submitted for review.  A great big thanks to all our alpha testers!  Your feedback has been invaluable and has truly helped us make the app much better than it would have been without you.  So what now?  Well, we intend to keep the alpha test group around, except now we're considering it the "beta channel".  It's still important to us, particularly since the update cycle through the AppStore is slow.  We'll still roll out updates in the same way that we have been.  We will also from time ask for targeted feedback on specific features.  And, lastly, if you're just now getting here and wished you had signed up for the alpha, feel free to sign up now anyway.  The form is still there, and we may still let in additional users to the beta channel depending on what our "active users" count looks like. Yesterday, one of our computers dropped to grub shell or honestly, I am unsure what shell it was when we turned on the machine.  It showed that it can't mount the root filesystem or something in this sense, because of inconsistencies.  I ran, I believe:  fsck -fy /dev/sda2   Rebooted and the problem was gone.  Here comes the question part:  I already have in her root's crontab:   @reboot /home/ruzena/Development/bash/fs-check.sh    while the script contains:  #!/bin/bash touch /forcefsck   Thinking about it, I don't know, why I created script file for such a short command, but anyways...  Further, in the file:  /etc/default/rcS   I have defined:   FSCKFIX=yes    So I don't get it. How could the situation even arise?    What should I do to force the root filesystem check (and optionally a fix) at boot?  Or are these two things the maximum, that I can do?  OS: Linux Mint 18.x Cinnamon 64-bit.  fstab:  cat /etc/fstab | grep ext4   shows:   UUID=a121371e-eb12-43a0-a5ae-11af58ad09f4    /    ext4    errors=remount-ro    0    1    grub:   fsck.mode=force    was already added to the grub configuration. Do I have to do any configuration to start brace expansion? When I run mkdir {1..10}, it just creates a dir naming {1..10}. How to lazy load images in ListView in Android Why can't a normal user `chown` a file? What, if anything should one do if a user's "about me" profile section contains extremely inflammatory terms (racist, etc.) I don't think anyone wants to be the thought police, and I'm particularly wary of censorship on a part of the site that is a user's personal self-statement, and is a little off the beaten path.  On the other hand, I'm also concerned that we may not want a site where clicking on a user to see their other posts would expose folks to racist or other bigoted terms that almost everyone would agree are inappropriate for civil discourse.    Don't forget the "be nice" rule".  Note that I don't want a "flag user" rule, which I know has been beaten to death already, and further think that swears and semi-inflammatory opinions, etc. should generally just be ignored - I'm only asking about terms that provoke serious emotions around racial, religious, ethnic, or sexual orientation issues.  What, if anything, is the appropriate response in such cases? Will I lose previous purchases when I change country in iTunes Double Negation in C++ What translation occurs when writing to a file that was opened in text mode that does not occur in binary mode? Specifically in MS Visual C.  unsigned char buffer[256]; for (int i = 0; i &lt; 256; i++) buffer[i]=i; int size  = 1; int count = 256;   Binary mode:  FILE *fp_binary = fopen(filename, "wb"); fwrite(buffer, size, count, fp_binary);   Versus text mode:  FILE *fp_text = fopen(filename, "wt"); fwrite(buffer, size, count, fp_text); I have a Storex eZee Tab 702 tablet. My Google Play Store was working very well until a few weeks ago, when it started saying,     Error while retrieving information from server [DF-DFERH-01]   I tried to clear the cache, clear the data, force stop, and downloading another Google Play Store, but I'm still having the same problem.  How can I fix this issue? Where do I ask where to ask? If I have a specific question I want to ask about, but I don't know which Stack Exchange site would be on topic for asking that question, where can I ask where to ask that question?  Is there any generic place where we can ask for advice? Is it here?  This is not about a specific question but in general.  For example: "Where should I ask for advice about fishing?", or if I need to know where I should ask "What would be the best fishing rod to fish for salmon?" (regardless of how good/bad these examples are) What is the best way to remove accents (normalize) in a Python unicode string? Is there a command to delete all the files in a directory that haven't been modified in N days?  I need to clean up some old logs. Improving "demonstrate a minimal understanding" close reason Note: the eventual outcome of this discussion involved dropping this particular off-topic reason, along with other changes. See:  &nbsp; &mdash;Shog     I was using the reason "demonstrate a minimal understanding" myself quite frequently, but the more I review, the more I have doubts about it and I think this reason can either be dropped entirely or at least rephrased, so I started to use "problems with code you've written must describe the specific problem" instead:     Questions asking for code must demonstrate a minimal understanding of the problem being solved.   So the poster has posted some description, maybe even a snippet of code and in his mind that already proves that he has that minimal understanding required. After all he described the problem and what he tried to solve. So telling him that he lacks that understanding is rather rude, and from the posters point of view doesn't even apply. What both version of the reason not emphasize is that a description of a problem is not various versions of     it doesn't work   but that we rather mean really specific details (see below). I can often see in the reopen review queue that the "improved" version just includes a more verbatim description, while what usually would be expected is some specific error message, a trace or a log, or whatever applies.  Then it continues with:     Include attempted solutions, why they didn't work, and the expected results. See also: Stack Overflow question checklist   comparing it to:     problems with code you've written must describe the specific problem — and include valid code to reproduce it — in the question itself. See SSCCE.org for guidance.   Which says essentially the same, only in more detail.   So the "minimum understanding" reason is IMO a more rude version of the reason telling the user that an SSCCE is missing.  The intent of giving this reason is to tell the user that he needs to give exact details to be able to help him, so IMO a checklist would be much better. Something like:     Your question was closed because it may not provide enough details. Things you can do to improve your question to get an answer include:         Format the code in a way that makes it readable and easily understandable.   Provide the smallest working sample of code demonstrating the problem (link to SSCCE)   Include your input and output.   If applicable provide logs (i.e. Android debugging log, stack trace, etc.).   If applicable provide the exact error messages (compiler, etc.).      And something like this should be best included already in the banner that appears when a question is closed instead of being hidden in some pages you often enough have to actively looking for.  It might be helpful to adjust this, based on the tags or close reason, as I realize that I'm more focused on the programming area, and the above list may not apply to other areas of SO.  Update  Another thing I forgot to mention (but wanted to) is this:     Questions asking for code   Is IMO essentially an invitation of the type of questions     I have a problem, can you provide the solution? (with an optional please)   while     Questions concerning problems with code you've written   puts at least some emphasis on the fact that the poster has to put some effort himself in trying to solve the problem by providing the code which he used to attempt the solution. Installed Teamviewer using a 64-bit system, but I get a dependency error Motion Tracking objects that go out of frame and re-enter later I am motion tracking a video in which the camera turns away from one scene and then shortly returns to it. However, the segments that went off-screen lose their track and do not continue tracking once the camera returns. I am wondering if there is a manual way to tell a segment where and when to continue looking for it's tracking target. Is this possible to do? When thinking about travelling, I often have to think about a place to visit, and then look up visa rules for that particular country. This can be a fairly painful process because official government sites can often be hard to find, and advice on (say) guidebooks or Wikitravel tends to be generic.  Is there any resource where I can enter my nationality/residency, and then look up a list of visa rules for various countries?