Validate Email Or Number in Regex
How to validate an email address in JavaScript
Intuitive explanation of entropy I have bumped many times into entropy, but it has never been clear for me why we use this formula:  If $X$ is random variable then its entropy is:  $$H(X) = -\displaystyle\sum_{x} p(x)\log p(x).$$  Why are we using this formula? Where did this formula come from? I'm looking for the intuition. Is it because this function just happens to have some good analytical and practical properties? Is it just because it works? Where did Shannon get this from? Did he sit under a tree and entropy fell to his head like the apple did for Newton? How do you interpret this quantity in the real physical world? To my (British) ear, it never sounds correct to say you have a doubt about something.  I expect that doubt to be pluralised, even if there's only one specific thing that I'm doubtful of, in one specific way, so I would always say "I have doubts about something".  Answers to  say that using doubt as a synonym for question is "Indian English". That seems to be the case in  which starts with "I have a doubt on my construction", because the OP really is saying "I have a question" (which he goes on to actually ask). Okay, it should have been a question/doubt about something, not on, but I'm not concerned with that detail here.  But in  the OP says "...there is a doubt that can I use [some construction]". In that case it's not easy to see how the word doubt can be replaced with question.  I don't have a problem with, for example, , because there I interpret it as doubt=uncertainty (a state of mind, syntactically equivalent to words like happiness, agitation, sorrow, etc.). But you certainly can't "have a happiness about something". So my question is...     Is it acceptable today to use singular noun doubt in any contexts where it means something other than state of mental uncertainty? If so, what are those contexts? Gnome Panel becomes fully transparent when window is not maximised! Execute bash commands over SSH while staying in interactive mode afterwards Probably it is a weird thing I would like to achieve but: I would like to SSH to a remote host and then execute some bash commands like "alias" automatically to use them afterwards in an interactive mode. The remote host does not allow the personal environment.    Is this possible? Would "expect" do it? Or is there any other way to achieve this? Basically I would like to have my bashrc without modifying any files on the remote host. When I learned about the Minkowski Space and it's coordinates, it was explained such that the metric turns out to be $$ ds^{2} = -(c^{2}dx^{0})^{2} +(dx^{1})^{2} + (dx^{2})^{2} + (dx^{3})^{2}   $$ where $ x^0,x^1,x^2,x^3 $ come from $ x^{\mu} : \mu = 0,1,2,3 $, and $ c $ is the speed of light. The first resource I had access to--I have to do a bit of digging for the exact paper--of course addressed that this invariant takes the tensor form: $$ ds^2 = g_{\mu \nu}dx^{\mu}dx^{\nu} $$ as well. This two things I've seen in all of my texts and online resources regarding resources. The element in question that varies between authors, is the time coordinate, $ x^0 $. When it was first explained to me, it was using the standard Cartesian representation for the spatial portion of the coordinates, and the time coordinate was labeled as $ x^0 = ict $. Squaring gives $ (x^0)^2 = -c^2t^2 $, and applying differential calculus, we get $ (dx^0)^2 = -c^2dt^2 $. Sensible and expected to have the tensor formula spit out the Minkowski Metric. The author then later explicitly states the coordinates are $ x^0 = ict, x^1=x,x^2=y,$ and $x^3 = z$.  My questions is then, why is it most authors on the subject omit the imaginary unit on the time coordinate? For example, .   The only reason I can fathom omission is if the author is using metric signature $ [+,-,-,-] $, where I started off learning the theory with signature $ [-,+,+,+] $ which may be the reason seeing the time coord with no imaginary unit seems dissonant to me. All help appreciated!  Edit: After reading the other answers, my questions is now one of why and how (mathematically) do we obtain the Minkowski Metric Signature. More specifically the one element with a different sign. How to reinstall yum? I made a big mistake: I removed yum. So now when I for example run the following it gives me an error:  $ yum clean all yum command not found.   I can download the package succesfully:  wget mirror.centos.org/centos/6/os/i386/Packages/yum-3.2.29-40.el6.centos.noarch.rpm --2014-01-15 15:42:59--  http://mirror.centos.org/centos/6/os/i386/Packages/yum-3.2.29-40.el6.centos.noarch.rpm Resolving mirror.centos.org... 192.133.139.3 Connecting to mirror.centos.org|192.133.139.3|:80... connected. HTTP request sent, awaiting response... 200 OK Length: 1019540 (996K) [application/x-rpm] Saving to: “yum-3.2.29-40.el6.centos.noarch.rpm.1”  100%[====================================================================================================================&gt;] 1,019,540    649K/s   in 1.5s  2014-01-15 15:43:01 (649 KB/s) - “yum-3.2.29-40.el6.centos.noarch.rpm.1” saved [1019540/1019540]   Trying to install the downloaded package gives the following error:  # rpm -ivh yum-3.2.29-40.el6.centos.noarch.rpm Preparing...                ########################################### [100%]     file /usr/lib/python2.6/site-packages/yum/__init__.py from install of yum-3.2.29-40.el6.centos.noarch conflicts with file from package yum-3.2.29-30.el6.centos.noarch [...] file /usr/share/yum-cli/yummain.pyo from install of yum-3.2.29-40.el6.centos.noarch conflicts with file from package yum-3.2.29-30.el6.centos.noarch   There is no directory "yum" in /usr/bin.  When I use the command "rpm -qa | grep yum".  yum-metadata-parser-1.1.2-16.el6.x86_64 yum-plugin-fastestmirror-1.1.30-14.el6.noarch yum-3.2.29-30.el6.centos.noarch   The distro is CentOS 6.3. How can I reinstall yum? I was reading about the equipartition theorem and I got the following quotations from my books:  A diatomic molecule like oxygen can rotate about two different axes. But rotation about the axis down the length of the molecule doesn't count. - Daniel V. Schröder's Thermal Physics. A diatomic molecule can rotate like a top only about axes perpendicular to the line connecting the atoms but not about that line itself. - Resnick, Halliday, Walker s' Fundamentals of Physics.  Why is it so? Doesn't the rotation take place that way? Story about space explorers who communicate but then decide they must destroy each other I'm looking for a sci-fi story or novel from many years ago where a deep space explorer encounters an alien explorer. The explorers learn to communicate; but on parting, each decides to destroy the other in order to protect his own world. Hide trivial comments how to properly save an architecture in a fpga ic forever ERD for a web development company I am working on an ERD which represents the relationships between entities in a Web development company behavioral model. There I have entities like:   Customer Service ----------- (like: web development, web design, web consultation, ...) Order-item ------- (like: web development, web design, web consultation, ...) Order Invoice Project Project-phase Task --------------- (like: web development task3, web design task7) Developer Delivery ----------- (like: the developed web app, web consultation session, ...)   I have this diagram so far, though I know it still has major problems, and it is just for a start point:    Main concept   Customer makes order-items on services, combined to form an order which relates to an invoice. Each project has multiple project-phases with each project-phase related to an order. Each project-phase has multiple tasks, each of which is assigned to a developer. Each project-phase has a delivery, which will be delivered upon payment of the related orders invoice.   Questions   Are has and contains completely the same? If not, how to choose between them for relationships?! what is the measure?! Are there any rational problems with the Main concept section? Any helpful ideas or suggestions to improve this model?   Any help with this, would be really appreciated.  Update 1  Refined version of the diagram     Update 2  Considering an analogy of a Fast food restaurant would be helpful:   Customer ---------- A hungry person Service ------------- Making a burger, Home delivery, Mobile delivery, ... Order-item --------- A burger, ... Order ---------------- A home-delivered burger Invoice -------------- The printed invoice Project -------------- Serving the customer for a complete year (like a contract) Project-phase ----- Serving the customer for a single day Task ----------------- Preparing the kitchen, process the meat, ... Developer ---------- The cookie Delivery ------------- A burger in wrapped in wax papaer and a drink with the tray   Update 3  ()     Project seems to be unrelated to Customer in your diagram. Is this what you intended?   I think Order is and Project should not be related, as I think it is something off-the-counter and customer has nothing to do with it. she/he is only after the delivery and is concerned about time and money, nothing else. Please tell if I am wrong.     Are all Orders part of a Project or can you have orders that aren't in a project at all?   All Orders are part of a project, except those new ones which are waiting in the process queue.     Similarly, you relate order item directly to customer, but also to order. Are orders split between customers?   Please think of Order-Item as these:   web design web design consultancy domain name consultancy ...   So, customer choose some of these items, collected as an Order which has an invoice and could be paid. Order-Items are directly related to offered Services. I don't understand what do you mean by Are orders split between customers?.     If not you don't need a ternary relationship between customer, order item and service. Rather, an order item is an intersection between order and service. Order items belong to customers by inheritance from order (or even project?)   This is somehow true: Order items belong to customers by inheritance from order, but I think it is more rational to say it like this:     Order-Items, inherited from Service, belong to Order which itself belongs to Customer. How to use unnumbered chapters with KOMA-script? I am having some issues with the hyperref package. I would like to create a Introduction chapter without a number so I use \chapter*{Introduction} and \addcontentsline{toc}{chapter}{Introduction} because I want it in the table of contents (see example).  But then the link in my PDF viewer gets it wrong and also the automatically created header is no longer correct (see picture). The table of contents shows the right page number (p.1) but the link is also wrong.   What is my mistake here?  \documentclass{scrbook}  \usepackage{hyperref} \usepackage{blindtext} \usepackage{glossaries}   \makeglossaries  \newacronym{eg}{e.g.}{% for example }   \begin{document}   \frontmatter  % abstract before the table of contents \chapter*{Abstract} \addcontentsline{toc}{chapter}{Abstract} \blindtext  \tableofcontents  % chapter with acronyms \cleardoublepage \phantomsection  \printglossary[type=\acronymtype] \addcontentsline{toc}{chapter}{Glossary}  \cleardoublepage \mainmatter   \phantomsection % \chapter{Introduction} % with this everything is correct but I don't want the number \chapter*{Introduction} \addcontentsline{toc}{chapter}{Introduction}  \blindtext  \pagebreak  \blindtext  % first chapter \chapter{Chapter1}  \gls{eg} \blindtext  \pagebreak  \blindtext   \end{document} Why is the Fisher Information matrix positive semidefinite? Received an email intended for another person I'm not really sure how email routing works, but someone ordered something on Amazon and I received the email instead of them.  Or maybe we both got it, I don't know.  The order doesn't show up in my account, so I'm certain I wasn't charged for it, but I shouldn't be getting other peoples' emails.  We'll say that my email is johndoe1@gmail.com, and somebody who's email is john.doe1@gmail.com places an order on Amazon.  The confirmation email is sent to me at johndoe1@gmail.com.  I checked the email header, and it did say To: john.doe1@gmail.com which is not my email address.  At first I thought that Google ignores periods in email addresses, but I tested the account setup and it doesn't give any error when you put a period in the address.  I didn't create the account; I just used the "check availability" function and the address I chose with a period was fine.  Maybe someone with knowledge about how Email works could tell me why this happened.  Is this a bug in the way Amazon sends emails?  Or is it a bug in how Google receives them?  Who should I report this issue to? This question is present as a matter of historical interest.  While you are encouraged to help maintain its answers, please understand that "big list" questions are not generally allowed on Ask Ubuntu and will be closed per the .   It is possible to greatly extend the usage time of a laptop or netbook running on battery by disabling various services and installing various packages.   What tricks or tips do people have for getting an extra hour or two out of their batteries? How can I know if $2^{2^{2^{2^{2}}}}+1=?$ is prime? How to reset the terminal properties and preferences? I fooled around with the colors and stuff, in the terminal, and now want to go back to the maroon and white default config of the the terminal. What files do I have to delete? How do I remove objects from an array in Java? Redirect the output using `sudo` I have a small question regarding using sudo with output redirect &gt;. To enable IP forwrding, someone can use the command:  echo 1 &gt; /proc/sys/net/ipv4/ip_forward   Executing this command will give permission denied as it requires root privileges. However, executing the same command with sudo gives also permission denied error! It seems that output redirect &gt; does not inherit the permissions of the preceding command echo. Is this right?  As a workaround I do:  echo 1 | sudo tee /proc/sys/net/ipv4/ip_forward   Is this the best way to do it? Am I missing something?  Please, note that this is an example and it applies to all commands that use output redirect. How to leave body of question empty?