I realized I don't understand a pretty basic thing about DNS. So, here's the flow  Assuming nothing is cached, When I open stackoverflow.com, the request goes to the root, then to the .com TLD registry, which in turn returns the name server (NS record) for the stackoverflow domain, i.e. cf-dns01.stackoverflow.com. Q1: Does the TLD only return only the NS records for the domain?  After that, the request goes to cf-dns01.stackoverflow.com., which has and returns the A record(s) (=IP(s)) of stackoverflow.com.  Q2: where is the IP address of cf-dns01.stackoverflow.com configured, i.e. how is the authoritative name server's IP resolved? Especially in this case, when the name server is a subdomain of the domain it has to resolve.
This is a  about DNS glue records.   What exactly (but briefly) is a DNS glue record?  Why are they needed and how do they work?
Integrals of Pullbacks I am trying to invite my son (IOS Pocket Edition) to my realms server (mac osx).   I keep getting the response "Could Not Invite The Provided Name" when I try to invite him. His name is all regular characters with no spaces . . . just letters and numbers.  Are the two game types incompatible, and if they are Is it possible to create the same type of 24/7 available server that is only open to him and other invitees like the OSX realms?   Thanks in advance for any clarification. Get month name from Date Are bridges like hubs or switches? It is my understanding that the main difference between a network hub and a network switch is that the hub always sends out all packets to all (other?) hosts while the switch is smart about it and keeps track of which port belongs to which MAC address via MAC address tables and whatnot. If a packet is addressed to a host which is connected to the switch but have never sent or received anything, the switch acts like a hub would. But every subsequent packet addressed to that host is not broadcast to the other hosts but forwarded only to that specific host. Please correct me if I got something wrong. Does a network bridge in Linux act like a hub would, or like a switch? Is it OK for OSB3 boards to get wet during construction? Is commitment necessary to participate in beta (as of June 2015)? There's this proposal for a new S.E. site which is about to enter the beta phase. It reads  This proposal is in: Commitment A successful site needs people to commit to use it. This proposal is 100% complete. Committed users will be invited to the private beta soon.  I'd like to participate in the beta but I don't want to commit (due to a full name being required). Will I be unable to participate in the beta if I don't commit? Restricting T to string and int? Rigid body simulation not interacting with objects that have an array modifier How to derive variance-covariance matrix of coefficients in linear regression I am reading a book on linear regression and have some trouble understanding the variance-covariance matrix of $\mathbf{b}$:    The diagonal items are easy enough, but the off-diagonal ones are a bit more difficult, what puzzles me is that  $$ \sigma(b_0, b_1) = E(b_0 b_1) - E(b_0)E(b_1) = E(b_0 b_1) - \beta_0 \beta_1 $$  but there is no trace of $\beta_0$ and $\beta_1$ here. XGBoost Loss function Approximation With Taylor Expansion As an example, take the objective function of the XGBoost model on the $t$'th iteration:  $$\mathcal{L}^{(t)}=\sum_{i=1}^n\ell(y_i,\hat{y}_i^{(t-1)}+f_t(\mathbf{x}_i))+\Omega(f_t)$$  where $\ell$ is the loss function, $f_t$ is the $t$'th tree output and $\Omega$ is the regularization. One of the (many) key steps for fast calculation is the approximation:  $$\mathcal{L}^{(t)}\approx \sum_{i=1}^n\ell(y_i,\hat{y}_i^{(t-1)})+g_tf_t(\mathbf{x}_i)+\frac{1}{2}h_if_t^2(\mathbf{x}_i)+\Omega(f_t),$$  where $g_i$ and $h_i$ are the first and second derivatives of the loss function.   What I'm asking for is convincing arguments to demystify why the above approximation works:  1) How does XGBoost with the above approximation compare to XGBoost with the full objective function? What potentially interesting, higher-order behavior is lost in the approximation?  2) It's a bit hard to visualize (and depends on the loss function) but, if the loss function has a large cubic component, then the approximation will likely fail. How is it that this doesn't cause problems for XGBoost? SF novel about one-way scientific expedition to inhabited planet, where telepathic aliens "talk" sort of ++like this++ and ~*~this~*~ I'm looking for a novel I read in the mid-to-late 1990s. It was a science fiction novel about an expedition of scientists sent at sub-light-speed (maybe in cryo-sleep?) to investigate a star system. Once there they set up a permanent scientific outpost.  There was at least one inhabited planet in the system, with a sentient species that communicated telepathically. The aliens' speech was represented with various symbols instead of quotation marks, with each named character having (I think) its own characteristic set of symbols, as in the title of the question. The scientists are eventually able to communicate with these (generally friendly) aliens.  The thing I remember most clearly is the end of the book. This was a one-way trip for the scientists, so gradually they all die off from various causes. When the next-to-last member of the crew dies of old age, the survivor is looking for an epigraph for her funeral, and (ironically, considering my current plight) can't find it in his search of the electronic library. He finally discovers it under "humor" because it's by Mark Twain: "Wherever she was, there was Eden." (This quote, of course, I found immediately in  and many other sources; it's from Eve's Diary.)  Other pieces of information I'm less sure of:   I don't think the planet was hospitable to humans; it may have been something like a gas giant, with the aliens "swimming" through the gas. I think the scientists stayed in an orbital space station, though they may have made expeditions to the planet in vehicles or protective suits. There may have been a "twin" planet, or a moon in an eccentric orbit, that caused periodic "flares" of some sort on the aliens' planet. There might have been some conflict between the crew early on, maybe because there was a non-scientific (military) commander, or perhaps just because people have conflict. I think the author was of sort of middling fame; respected and known by SF enthusiasts, but not a superstar like Asimov or Clarke. Almost certainly male, and probably with a professional science background. I think the science was fairly authentic, although the first-contact bit was obviously highly speculative. I've been going through  and have a question on the inductive proof for the . Recurrent relation:  $J(1) = 1$ $J(2n) = 2J(n) - 1$ $J(2n+1) = 2J(n) + 1$  Closed form hypothesis:  $J(2^m + l) = 2l + 1$, for $m \ge 0$ and $0\le l&lt;2^m$  Inductive Proof (for the even case):  $J(1)=1$ $J(2^m+l)= 2J(2^{m-1}+l/2)-1=2(2l/2+1)-1=2l+1$  How do the authors get from $2J(2^{m-1}+l/2)-1$ to $2(2l/2+1)-1$? In C# why can't a conditional operator implicitly cast to a nullable type I have several std::vector, all of the same length. I want to sort one of these vectors, and apply the same transformation to all of the other vectors.  Is there a neat way of doing this? (preferably using the STL or Boost)?  Some of the vectors hold ints and some of them std::strings.  Pseudo code:  std::vector&lt;int&gt; Index = { 3, 1, 2 }; std::vector&lt;std::string&gt; Values = { "Third", "First", "Second" };  Transformation = sort(Index); Index is now { 1, 2, 3};  ... magic happens as Transformation is applied to Values ... Values are now { "First", "Second", "Third" }; C structure initialization? Determinant of a non-square matrix I wrote an answer to  question based on determinants, but subsequently deleted it because the OP is interested in non-square matrices, which effectively blocks the use of determinants and thereby undermined the entire answer. However, it can be salvaged if there exists a function $\det$ defined on all real-valued matrices (not just the square ones) having the following properties.   $\det$ is real-valued $\det$ has its usual value for square matrices $\det(AB)$ always equals $\det(A)\det(B)$ whenever the product $AB$ is defined. $\det(A) \neq 0$ iff $\det(A^\top) \neq 0$   Does such a function exist? Why is $\Gamma\left(\frac{1}{2}\right)=\sqrt{\pi}$? $\sqrt{7\sqrt{7\sqrt{7\sqrt{7\sqrt{7\cdots}}}}}$ approximation How to distribute the objects on a sphere evenly? I want to replicate this mesh -    I think the best way is using a particle system of cubes. So I need to create a sphere of simple circles. It is necessary that in each circle the vertices are at equal distance from each other. But I don't know how to create those circles. Please add TeX rendering in titles on the iOS app TeX support works fine in question and answer bodies, and comments can be rendered manually by tapping and selecting Render MathJax. See related question .  However, the titles are not rendered which makes the browsing experience less than perfect. See screenshot below.   Are there any plans to implement MathJax support in titles?  (I'm running Stack Exhange iOS App Version 1.0.0 on an iPhone 4S with iOS 7.1.1)    EDIT: I was just informed about  asking for the same functionality for the Android app. I guess I shouldn't hope too much then...