Just wanted to know what company does SO gets its hosting services from. The site is very responsive and fast. I would like to get my hosting done in the same place. Thanks
What tools and technologies are used to build the Stack Exchange Network?  See also:
BibTeX loses capitals when creating .bbl file Solving $\cos x=x$ I would like to know how can we solve the equation $\cos x = x$, without graphing. I know that  there would only be one solution, that is obvious, that too in between $0$  and $\frac{\pi}{2}$. Is there any real expression in finite terms [perhaps we call that closed form, I am not sure] that could give $x$ or $\cos x$. Although I have not studied Taylor series, I know that it only gives an infinite series, which is not what I want. I suspect that it could not be done, but can anyone explain me why?  Just for completeness, Wolfram Alpha, gives the approximate answer $x = 0.7390851332151606416553120876738734040134$, but fails to give a exact solution. Why am I getting authentication errors for packages from an Ubuntu repository? I'm experimenting with VPNs, so I wanted to install pptpd.  john@desktop:~$ sudo apt-get install pptpd [sudo] password for john:  &lt;snip&gt; The following extra packages will be installed:   bcrelay The following NEW packages will be installed:   bcrelay pptpd 0 upgraded, 2 newly installed, 0 to remove and 11 not upgraded. Need to get 90.5 kB of archives. After this operation, 442 kB of additional disk space will be used. Do you want to continue [Y/n]? y   And everything runs fine until I get down to here:  WARNING: The following packages cannot be authenticated!   bcrelay pptpd Install these packages without verification [y/N]? n E: Some packages could not be authenticated   No authentication?  I'm no security expert, but I'm fairly sure that's a bad thing.  So I .  john@desktop:~$ apt-cache policy pptpd pptpd:   Installed: (none)   Candidate: 1.3.4-3ubuntu1   Version table:      1.3.4-3ubuntu1 0         500 http://us.archive.ubuntu.com/ubuntu/ natty/main i386 Packages john@desktop:~$ apt-cache policy bcrelay bcrelay:   Installed: (none)   Candidate: 1.3.4-3ubuntu1   Version table:      1.3.4-3ubuntu1 0         500 http://us.archive.ubuntu.com/ubuntu/ natty/main i386 Packages   They're coming from Ubuntu.com and not being properly authenticated?  What's going on here? I've just upgraded my computer hardware(cpu + motherboard + graphic card + memory + hard disk), so that install a new OS is needed. I tried to download debian-6.0.6-amd64-netinst.iso with wget command but the speed is so slow that I could not bear. 4Kb/s ~ 17 Kb/s, slow like a running turtle, or even more slower if I use Chrome.  I've read the help information of wget, it seems like there are no options could make it more faster.  Is there anyway to make wget faster? Or is it possible to make it multi-threading download?  PS: my bandwidth is 4M. I use this command:  wget -c url  http://hammurabi.acc.umu.se/debian-cd/6.0.6/amd64/iso-cd/debian-6.0.6-amd64-netinst.iso How can I limit ssh *remote* port forwarding? I need to limit which ports can be remotely 'ssh -R' forwarded by an user.  I know about permitopen option on authorized_keys, but as it says on man page it only limits local 'ssh -L' port forwarding  As discussed  a user would get the same with netcat or similar, but in this case user has no shell access  I also found  that talks about using selinux or LD_PRELOAD, but I never configured selinux before and can't find info on how to do that with LD_PRELOAD.  maybe someone have made a patch for openssh to implement that?  EDIT: I've found  so I guess it's not yet implemented Why might one also use a blank constructor? I was reading some Java recently and came across something (an idiom?) new to me: in the program, classes with multiple constructors would also always include a blank constructor. For example:   public class Genotype {   private boolean bits[];   private int rating;   private int length;   private Random random;    public Genotype() {              //  &lt;= THIS is the bandit, this one right here     random = new Random();   }    /* creates a Random genetoype */   public Genotype(int length, Random r) {     random = r;     this.length = length;     bits = new boolean[length];      for(int i=0;i&lt;length;i++) {         bits[i] =random.nextBoolean();     }   }    /* copy constructor */   public Genotype(Genotype g,Random r) {     random = r;     bits = new boolean[g.length];     rating = g.rating;     length = g.length;      for(int i=0;i&lt;length;i++) {         bits[i] = g.bits[i];     }    } }   The first constructor doesn't seem to be a "real" constructor, it seems as though in every case one of the other constructors will be used. So why is that constructor defined at all? I have 2 graphics cards on my laptop. One is IGP and another discrete.   I've written a shell script to to turn off the discrete graphics card.  How can I convert it to systemd script to run it at start-up? I have fought the pumpkin moon and died like 3 times and made it to wave 7 or 6 whenever the pumpking first appears. I have full turtle armor and my primary melee weapon is a demon sickle. I also have the possessed hatchet, magnet sphere, and sniper rifle. I also have other gear like the Uzi. What should I replace, what should I take out, and what should I add? Thanks! Increment Record based on field I was tasked to prove that when given 2 graphs $G$ and $\bar{G}$ (complement), at least one of them is a always a connected graph.  Well, I always post my attempt at solution, but here I'm totally stuck. I tried to do raw algebraic manipulations with # of components, circuit ranks, etc, but to no avail. So I really hope someone could give me a hint on how to approach this problem. How to solve this equation $x^{2}=2^{x}$? How to solve this equation $$x^{2}=2^{x}$$   where $x \in \mathbb{R}$.  Por tentativa erro consegui descobri que $2$ é uma solução, mas não encontrei um método pra isso. Alguma sugestão?(*)  (Translation: By trying different values I've found that $2$ is a solution, but I couldn't find any method to this though. Any suggestions? ) access parent object in javascript Sums and products of algebraic numbers How does one go about proving that the sums and products of two algebraic numbers over a field $F$ (say $a,b\in K$, where $K/F$ is a field extension) is also algebraic?   If we call $f_a$ and $f_b$ the min. poly's of $a$ and $b$, then I'm assuming the answer involves such polynomials. Perhaps looking at their roots in splitting fields for both of them? And finding a "big" splitting field, constructed from those two other ones?      In particular, I'd like a way of explicitly constructing the minimal polynomials $\ f_{ab}$ of $ab$ and $f_{a+b}$ of $a+b$.     I read somewhere that $g(x)=\Pi_j\Pi_i (x-\alpha_i\beta_j)$ works for $ab$, where the $\alpha_i$ and $\beta_j$ are the roots of $f_a$ and $f_b$, respectively, but I do not know why $g(x)\in F[x]$.  Similar remarks for $a+b$ I would like to know where gravity gets its energy to attract physical bodies?  I know that the law of conservation states that total energy of an isolated system cannot change. So gravity has to be getting its energy from somewhere, or else things like hydropower plants wouldn't be able to turn the power of the falling water into a spinning rotor.  Just to be clear, Lets create an example:  Lets say we have two objects with equal mass close to each other. So gravity does its job and it pulls each other closer, this gets turned into kinetic energy. This is where I'm lost. According to the law of conservation energy can't be created or destroyed and the kinetic energy comes from the gravitational pull so where does the gravitational pull gets its energy.  If that energy isn't being recycled from some where else then that means you have just created energy, therefore breaking the law of conservation. A lot of missing dependencies when I try to install all software I have problem with missing dependencies, when I try to install some software by apt-get and also from downloaded .deb files.  I am using Debian 10. After installation I removed a lot of software which I no needed eg. Deluge, some terminals (I have only LXTerminal now), some tools from Libre Office Packet etc.  Now, when I try to install Atom, TeamViewer, or some RDP tools, every time I got information about missing unresolved dependencies, common it is libgconf, libcurl etc.  How can I resolve this issue? Now I can not install a lot of must have software, and when I try to apt-get install this packets, I got error, that no libs with this name were found.  Here is my sources.list. Maybe it will be some tip to fix this problem:  deb http://security.debian.org/debian-security/ buster/updates non-free contrib main deb-src  deb-src http://security.debian.org/debian-security/ buster/updates non-free contrib main deb-src   deb http://deb.debian.org/debian/ buster-backports main contrib non-free  deb-src http://deb.debian.org/debian/ buster-backports non-free contrib main   deb http://ftp.de.debian.org/debian stretch main contrib non-free deb-src http://ftp.de.debian.org/debian stretch main contrib non-free   Stretch repos were added for test purposes, but when I comment them, it looks the same. How can I prove the formula for calculating successive entries in a given row of Pascal's triangle? I've found in Wikipedia the formula for calculating an individual row in Pascal's Triangle: $$v_c = v_{c-1}\left(\frac{r-c}{c}\right).$$ where $r = \mathrm{row}+1$, $c$ is the column starting from $0$ on left and $v_0 = 1$.  Now, I've tried to do by hand and it works,  but I don't understand how to find this magic formula when I don't have access to Wikipedia...:) Why does speed of light have to be constant? My question is not about why speed of light has a particular constant value which has been sufficiently addressed in other questions on SE already. I want to know whether the fact that speed of light has to be constant is a consequence of other fundamental laws of Universe, or is it just a fundamental law itself.  My motivation for asking this question is thinking about Special Theory of Relativity. One way of understanding why time dilation and length contraction happen is to look at it from the perspective of constancy of speed of light. Alice who is on the moving source of light and Bob, a fixed observer see the speed of light as constant. In order for Alice to see the speed of light as a constant value, time slows down and length expands (in forward direction) thus keeping the speed constant.   In the  I'm doing, the Professor tells that this constancy of speed of light could be more generally referred to as, "Principle of wave constancy". That is, the speed of any wave is a property of the medium (and thus it remains constant). So, the speed of a wave in water, for example, remains constant even if the source of the waves (say a paddle keeps moving). Frequency and wavelength may change but the speed remains constant. In a similar fashion, light is an electromagnetic wave and thus its speed is constant. And the constant speed is defined by the medium (which happens to be vaccum i.e., no medium for light).   My questions are twofold now -    Is the above reasoning correct to explain why speed of light is constant? (i.e., thinking of it as constancy of any wave in general) Theoretically, could there be other waves that could travel through vaccum at a different speed? If yes, time dilation and length contraction will have to be different.   I'd appreciate it if the answer is kept non-technical (essential math is okay). More importantly, I request you to try and explain without using General Theory of Relativity. Because I'm thinking how the scientific understanding by 1905 had already knew that speed of light has to be constant. How can I respawn the Ender Dragon? I have a server and the Ender dragon has been defeated.  However, returning to The End does not respawn the Ender Dragon.  How can I get the final boss back on a vanilla server without using mods? How can the Next Generation's unfamiliarity with the Borg be explained after Archer's encounter? In  Starfleet learns about a culture that we (as audience) know as The Borg. According to TNG, the Federation doesn't seem to know anything about these beings when . Although Archer's crew doesn't learn the name of the Borg, it would be reasonable that the Federation would inherit the information about such a menacing enemy and it would also be reasonable to assume the crew of the Enterprise-D to be able to match their encounter with their databases' information.  Obviously, out-of-universe the explanation is that the scripts weren't written in chronological order, but how can the apparent obliviousness about the Borg be explained in-universe? Do I need an ESTA if entering the US from Canada by train As a UK citizen, I will be flying to Vancouver (for which I will need an eTA) and then taking an AMTRAK train to Seattle. I will stay in Seattle for several days before returning to Vancouver and flying back to the UK. Do I need an ESTA for entering the US during this trip?