I cannot correlate with closure, negative votes, no answers, or any common thread between questions that are grayed out, and those that are not.  One can still open them, and answer or comment at will.    Why do some questions have a different appearance?
I just refreshed the [iphone]-tag-site, and I'm seeing all kinds of questions with what appears to be a grey overlay (see image). What is this for/why am I seeing this? I have never seen anything like this before..
Removing control chars (including console codes / colours) from script output Proving $\sum_{k=1}^n{k^2}=\frac{n(n+1)(2n+1)}{6}$ without induction I was looking at: $$\sum_{k=1}^n{k^2}=\frac{n(n+1)(2n+1)}{6}$$  It's pretty easy proving the above using induction, but I was wondering what is the actual way of getting this equation? Linux Bulk/Remote Administration Beside our internal IT infrastructure, we've got around 500 Linux machines hosting our services for the on-line world. They are grouped in a bunch of clusters like Database A-n, Product A-n, NFS, Backoffice and so on. Furthermore, they are administered by an external provider, according to our specifications and requirements.  However, we face a lot of trouble during (web-) software development, roll-out and deploy - especially because the dev- and staging-environments have almost nothing in common with the live systems (I spare out the nasty details..).  Thus, I've tried to create virtual machines, copied the various live-systems as exactly as possible and prepared them to connect to e.g. the development-databases instead of the "real" ones transparently for developers (they aren't root). This works pretty well, but...  I was wondering how one could administer those systems remotely and in bulk? Is there some software family I'm not aware of? Or, at least, some techniques or principles one should be familiar with?  We would provide every developer with a bunch of images to be run locally (VirtualBox). The QA dept. would get virtual clusters (XEN or Hyper-V). If I need to provide an additional server-module, re-route a new database connection or just want to update everything provided by the package manager... how could I possibly do that without being forced to log on to every system and/or ask my colleagues to download and run some fixture-script?  I believe there are plenty of solutions. Well, somehow I'm too stupid to enter the correct keywords into the search engines... Or isn't this issue as trivial as it sounds?  For the record:   Almost all systems are running Debian GNU/Linux 6.x "squeeze" No developer is forced to use a particular OS at his/her workstation The budget is limited, of course, but not too small to buy proprietary software A solution that would involve our aforementioned provider is preferred How to create a link to a URL provided in a bib file with custom text Using biblatex, how can I get a URL from the bib file so that I can use it in the \href command?  I am guessing that the \usefield command is what I am looking for and that in order to use that I would need the \entrydata command. But both are undefined.  \documentclass{article} \usepackage[backend=biber]{biblatex} \usepackage{hyperref}  \usepackage{filecontents} \begin{filecontents*}{sources.bib} @online{biblatex,     url = {http://mirrors.ctan.org/macros/latex/contrib/biblatex/doc/biblatex.pdf},     urldate = {2018-02-02}, } \end{filecontents*}  \addbibresource{sources.bib}  \begin{document}      % This would work, but I don't want to duplicate the URL from the bib file:     \href{http://mirrors.ctan.org/macros/latex/contrib/biblatex/doc/biblatex.pdf}{biblatex documentation}      % Does not work because \citeurl{biblatex} is already a link, not the URL:     %The \href{\citeurl{biblatex}}{biblatex documentation}      % Does not work because \usefield and \entrydata are undefined:     %The \entrydata{biblatex}{\usefield\href{url}{biblatex documentation}}      \printbibliography \end{document} Direction of friction for object that rolls with slipping How do I transfer a file to my server using PuTTY? What differences exist between third-party and official lenses for Canon APS-C DSLRs? I am just another guy who just taken up photography as a social hobby who enjoys taking pics of everyday life, social events / parties &amp; some portraits. Need camera for short overseas trip (2-3 times a year), maybe some landscape pics.   I have done some research and have come up a list of main lenses that I am considering and hope you guys can offer some advice / comments to whether I should make those purchases.   Main reason why I chose "third party lenses" 'cos apparently they cost less, newer and are "award winning" - so I don't see the point of spending more for the original canon lens (which some maybe quite old - ie 3-5 yrs).   Sigma 17-70mm F2.8-4 DC MACRO OS HSM Lens (day to day lens) Sigma 85mm F1.4 EX DG HSM (for portrait &amp; bokeh effects) Wide angle lens (any suggestions)   Main questions / comments please...   Q1) What do you think of the lens choices that I have decided to build my kit?  Q2) Any wide angle lens that you will recommend? (third party or canon) or should I get wide angle zoom lens ? Q3) What are your thoughts that I chose all my lens from Third Party instead of Official Canon ones... is that a "smart move"? Is the set $S=\left\{\left(z_1,z_2\right)\in \mathbb C\times \mathbb C:z_1^2+z_2^2=1\right\}.$ compact? Consider the set $$S=\left\{\left(z_1,z_2\right)\in \mathbb C\times \mathbb C:z_1^2+z_2^2=1\right\}.$$  Is this set compact in $\mathbb C^2$ ?  As $\mathbb C^2$ is a finite dimensional space so a subset of it is compact if and only if it is closed and bounded.  But $z_1^2+z_2^2=1$ gives $(x_1^2+x_2^2)-(y_1^2+y_2^2)=1$ and $x_1y_1+x_2y_2=0$ , where $z_1=x_1+iy_1$ and $z_2=x_2+iy_2$.  But from here how I can show that the set is closed and bounded ? I'm writing a simple bash script, but I need it to check whether it's being run as root or not. I know there's probably a very simple way to do that, but I have no idea how.  Just to be clear: What's a simple way to write a script foo.sh, so that the command ./foo.sh outputs 0, and the command sudo ./foo.sh outputs 1? Books for particle physics and the Standard Model Seems that I just proved $2=4$. Given a recurrence relation:   $$ x_{n+2} = px_{n+1} + qx_n + r $$   Find the general term $x_n$ given initial conditions $x_1 = a$ and $x_2 = b$, where $a,b,p,q,r$ are some given numbers, $n \in \mathbb N$.   Since this is a non-homogeneous linear recurrence relation i've started with solving for $x_n^h$ which is a solution for homogeneous relation:  $$ x_{n+2} - px_{n+1} - qx_n = 0 \\ \lambda^2-p\lambda - q = 0 $$  I'm assuming here that two distinct roots $\lambda_{1,2}$ exist for this equation (single root case is handled similarly). So the general term is in the form:  $$ x_n = C_1\lambda_1^{n-1}+C_2\lambda_2^{n-1} $$  With the initial condition it's possible to find the values for $C_1$ and $C_2$:  $$ C_1 + C_2 = a \\ C_1\lambda_1 + C_2\lambda_2 = b $$  After solving the system of equation with respect to $C_1$ and $C_2$:  $$ C_1 = \frac{b-a\lambda_2}{\lambda_1 - \lambda_2} \\ C_2 = \frac{b-a\lambda_1}{\lambda_2 - \lambda_1} \\ $$  Which eventually results in:  $$ x_n^h = \frac{(a\lambda_2 - b)\lambda_1^{n-1} - (a\lambda_1-b)\lambda_2^{n-1}}{\lambda_2 - \lambda_1} $$  Final solution consists of a sum of particular and homogeneous solutions, but that is where I got stuck. What is the way to find the particular solution for the given problem? I'm pretty new to such kind of problems so any details are very much appreciated.  upd:  Based on the technique in @rtybase answer here are my findings:  $$ \begin{align} x_1 &amp;= a \\ x_2 &amp;= b \\ x_3 &amp;= pb - qa + r \end{align} $$  Writing the system for $c_1, c_2, c_3$:  $$ \begin{align} a &amp;= c_1 + c_2 + c_3 \\ b &amp;= c_1\lambda_1 + c_2\lambda_2 + c_3 \\ pb+qa+r &amp;= c_1\lambda_1^2 + c_2\lambda_2^2 + c_3 \end{align} $$  Now solving for the coefficients gives a huge formula. The answer to this problem suggests that:  $$ x_n = \frac{(\lambda_2(a+\gamma) - b -\gamma)\lambda_1^{n-1} - (\lambda_1(a + \gamma) - (b-\gamma))\lambda_2^{n-1}}{\lambda_2 - \lambda_1} - \gamma $$  Where:  $$ \gamma = \frac{r}{p+q-1} $$  Unfortunately I was not able to arrive at exactly the same result, but testing my solution (not posting since it's monstrously big) using SameQ in Mathematica with the given answer shows that they are equivalent. Those transformations are not very obvious to me, so i skipped them since the answer matches the given one (yet the symbolic expressions are different) Can I publish a feed of my Meetups to appear on my Google Calendar? I'm a bit of a Meetup junkie so I'd like to automate the appearance of any Meetup I RSVP to automagically appear on my Google Calendar. What is needed for a game's stats to be saved? Last night I played a few games with my friend. While in the last game, I suddenly remembered something I had to do and was about to leave but my friend told me that I would lose my progress if I did so. I stayed until the end, through the scoreboard screen and quit while the next map was loading. Taking a look  now, I can't see any of yesterday's games. I don't remember how much XP I had yesterday so there is no sure way for me to check that. I had gotten a dozen ribbon and I don't see them in the feed, but there could be specific conditions to be showing them.  My questions then.   If you quit a game early, do you lose progress / XP? What if you quit on the scoreboard? Do you need to wait for the next map to load to save your progress? Any idea why the games aren't showing up? Some of them I definitely waited for the next map to load. Is there a delay for them to be shown on Battlelog?   Thanks. I'm loading an XML document in my C# application with the following:  XDocument xd1 = new XDocument(); xd1 = XDocument.Load(myfile);   but before that, I do test to make sure the file exists with:  File.Exists(myfile);   But... is there an (easy) way to test the file before the XDocument.Load() to make sure it's a valid XML file?  In other words, my user can accidentally click on a different file in the file browser and trying to load, say, a .php file causes an exception.    The only way I can think of is to load it into a StreamWriter and simple do a text search on the first few characters to make sure they say "  Thanks!  -Adeena I am considering trying to setup a  system. It seems like this would be pretty straight forward if I had two graphics cards or if I didn't need hardware based OpenGL acceleration. Is it possible to setup a multi seat system with only a single dual-headed graphics card? I'm installing a brand new motion activated security light to an existing box on my garage. After removing the old light, I notice it only has white/black wiring while the new light fixture has of course white/black/green (ground) wires to attach. What do I do if no ground wire is available with the old box? Different voltage sources in parallel I realize connecting two different voltage sources in parallel is a contradiction (in an ideal circuit). But if I were to connect this in practice and measure the voltage across points A and B, what value of voltage would it show? And how much current would be sunk by the 5V battery? (Non ideal conditions) Writing ISO 19139 compliant XML metadata from spreadsheet? I am looking at a way to migrate the metadata from a spreadsheet into ISO 19139 compliant XML metadata documents for a collection of shapefiles. I can parse the data out of the spreadsheet into strings without issue however I am getting lost on how to take those strings and use them to populate the elements.   This example:  covers most of what I want to do, however I would need to generate the tree beforehand based on the ISO 19139 schema. PyXB seems to have schema bundles for OpenGIS but I would like some insight on how to connect everything. Find all solutions of the equality $y^2=x^3+23$ for integers $x,y$   I guess, that x cannot be even. because we can apply mod 4 test  say $x=2k$ then $y^2=8k^3+23\equiv3\mod(4)$   but, this is not possible, since the square of an integer is congruent to $0$ or $1$ $\mod (4)$  so $x$ is of the form $4k+3$ or $4k+1$  $x=4k+3$ is also not possible for the same reason but for the last case the test fails.  any hints ?