When you enter a password and it's correct the response is practically instantaneous (i.e. the log on process).  When you enter an incorrect password (accidently, forgotten, etc) however, it takes a while (10-30 seconds) before it responds that the password was incorrect.  Why does it take so long (relatively) to say "incorrect password"?  This has always bugged me about entering incorrect passwords on Windows and linux (real and VM); I'm not sure about mac OSX as I can't remember if it's the same, been awhile since I last used a mac.  EDIT: for the sake of duplication, I'm asking in the context of a user logging on to the system at the physical computer rather than through ssh which could conceivably use somewhat different mechanisms to log in/validate credentials.
The most prominent place I've noticed this is when SSH-ing at work, but I feel like I've observed this behaviour elsewhere too.  When I try to log into Linux servers from my Windows desktop at work, I've noticed that if I mis-type my password, it takes a good 5 seconds or so before I get "Access Denied" back. Then when I type my password correctly, the login (along with welcome messages etc) is virtually instant.  Is there any logical reason for this, or would it be down to some odd configuration that's specific to the machines here at work?
In ArcMap, when you export data to PDF it preserves the layering within the PDF so that you can turn on/off individual map layers (within the PDF itself).  When I export from QGIS to PDF, I don't appear to have the option for this.  Does QGIS support preserving layers when exporting to PDF?  Is there a special setting needed to enable this capability?  Or has anyone wrote a plugin for this feature? I'm granted a one-entry Schengen visa and now I can't go. Will having an unused visa in my travel passport have any consequences like future visas refusal? Do I need to do anything special? How to prove this inequality $\left|x\sin{\frac{1}{x}}-y\sin{\frac{1}{y}}\right|<2\sqrt{|x-y|}$? For any real numbers $x,y\neq 0$,show that $$\left|x\sin{\dfrac{1}{x}}-y\sin{\dfrac{1}{y}}\right|&lt;2\sqrt{|x-y|}$$ I found this problem when I dealt with . But I can't prove it. Maybe the constant $2$ on the right hand side can be replaced by the better constant $\sqrt{2}$? Thank you. Is there a route that would allow travel to the US from the UK or Schengen area, given the US travel ban? I'm a European citizen living in the US on a work visa. Due to some unfortunate circumstances, I need to urgently visit my home country. However I would like to get back to the US after my visit is complete. Flying from the US to Europe is not a problem as there are still daily flights between some city pairs. However going back could be tricky, as  anyone who has been in the Schengen area in the past 14 days from re-entering the country.   Is there a flight route that would let me from Europe to some country, stay there for 14 days and then fly to the US? Canada is out, as they've banned international visitors. Mexico seems to keep their border open, but I  any direct European flights from their airports. Almost every other country either suspended flights or doesn't allow foreign nationals to enter. Can I run Android apps on Ubuntu? How do I make Bash my default shell on Ubuntu? What is the most preferred and easiest way to do pagination in ASP.NET MVC?  I.e. what is the easiest way to break up a list into several browsable pages.  As an example lets say I get a list of elements from a database/gateway/repository like this:  public ActionResult ListMyItems() {     List&lt;Item&gt; list = ItemDB.GetListOfItems();     ViewData["ItemList"] = list;      return View(); }   For simplicity's sake I'd like to specify just a page number for my action as parameter. Like this:  public ActionResult ListMyItems(int page) {    //... } Can somebody tell me why the counting measure (so, if $S=P(X)$, then $\mu(A)$=infinity if $A$ isn't finite and $\mu(A)=$#$A$ if $A$ is finite) is a measure? (The second property of a measure isn't clear for me). How to call alert after all ajax requests are done? When I execute a program without specifying the full path to the executable, and Bash must search the directories in $PATH to find the binary, it seems that Bash remembers the path in some sort of cache. For example, I installed a build of Subversion from source to /usr/local, then typed svnsync help at the Bash prompt. Bash located the binary /usr/local/bin/svnsync for "svnsync" and executed it. Then when I deleted the installation of Subversion in /usr/local and re-ran svnsync help, Bash responds:  bash: /usr/local/bin/svnsync: No such file or directory   But, when I start a new instance of Bash, it finds and executes /usr/bin/svnsync.  How do I clear the cache of paths to executables? Irreducible polynomial which is reducible modulo every prime How to show that $x^4+1$ is irreducible in $\mathbb Z[x]$ but it is reducible modulo every prime $p$?   For example I know that $x^4+1=(x+1)^4\bmod 2$. Also $\bmod 3$ we have that $0,1,2$ are not solutions of $x^4+1=0$ then if it is reducible the factors are of degree $2$. This gives that $x^4+1=(x^2+ax+b)(x^2+cx+d)$ and solving this system of equations $\bmod 3$  gives that $x^4+1=(x^2+x+2) (x^2+2x+2) \pmod 3$. But is there a simpler method to factor $x^4+1$ modulo a prime $p$? Often in fantasies involving magic casting spells requires a certain physical or mental toll on the caster. I don't recall any mention of spells being physically or mentally taxing in the Harry Potter universe.  Does this mean they can cast an unlimited number of spells in a given day without any negative effect to themselves? What is the logical form of the definition of validity? My text book gives the definition of validity as "An argument is valid IFF if the premises are true, then the conclusion must be true". Using a conditional elimination on the RHS of the IFF yields "An argument is valid IFF the premises are false or the conclusion is true".   Yet, this seems strange to me. Am I doing something incorrect with my manipulation or is this definition actually true? Surely the criteria for a valid argument goes beyond a false set of premises or a true conclusion. How can I manage the "Open with Explorer" button based on user permissions? Beamer - create subsubsection page Creaking from cranks/spindle. How to fix? Why is there free space at the end of NTFS Partitioned Drives? I've been wondering this for a while, why is there free space at the end of a drive that was formatted to NTFS in windows?  You can only see the unallocated space in Linux. So why is it there? Does it serve a purpose? I've just updated to macOS 10.12 and when I try to execute srm in Terminal, it says -bash: srm: command not found.   Is srm removed? How to fix this? Lighter nuclei liberate energy when undergoing fusion, heavier nuclei when undergoing fission.  What is it about the nucleus of an Iron atom that makes it so stable?  Alternatively: Iron has the greatest nuclear binding energy - but why? How do I restore the sources.list file? I accidentally removed my /etc/apt/sources.list file. Is it hosted by Canonical somewhere so I can re-add it to the system? It's a bit of an emergency, so I hope someone can provide a quick answer.