This  was last seen 19 minutes ago, yet he asked  13 minutes ago.
I posted what I think is a clever answer to:    But then I noticed the question was posted a while ago on Mar 2.  I wanted to see what the chance of the user logging back in and seeing my answer, but the  says the user was seen on Feb 19.  I thought this was supposed to be the date they last logged in? How could they have asked a question on Mar 2 if they haven't logged in since February?  Thanks
Parse error in sudoers file Viewshed analysis plugin on QGIS keeps resulting in errors I've been trying to test the Viewshed analysis plugin on QGIS 2.10, but it keeps giving back error messages. I've tried to reduce the radius area and have also reprojected both my DEM and my point shapefile to UTM, in order to avoid a possible memory error mentioned in another question regarding this plugin (that the using of geographic coordinates might cause a memory error).  Still, I keep getting Python error messages when I try to run it. Does anyone know whats going on? Is this plugin "broken"? What is some good free software for doing physics calculations?  I'm mainly interested in symbolic computation (something like Mathematica, but free). If $a^3 =a$ for all $a$ in a ring $R$, then $R$ is commutative. Let $R$ be a ring, where $a^{3} = a$ for all $a\in R$. Prove that $R$ must be a commutative ring. Significance contradiction in linear regression: significant t-test for a coefficient vs non-significant overall F-statistic A No-Nonsense Introduction to Quantum Field Theory I found Sean Carroll's "A No Nonsense Introduction to General Relativity" (about page .  pdf ), a 24-page overview of the topic, very helpful for beginning study.  It all got me over the hump of learning the meaning of various terms associated with GR, most of which I had heard before without understanding.  It also outlined the most important examples.  Is there a similar document for quantum field theory, which presents the main equations, briefly describes the main ideas, and summarizes the most important applications and results so that the reader can feel the lay of the land before studying in depth? Can US citizens go to Cuba freely and vice versa after the recent diplomatic relation restoration? A recent full diplomatic relation restoration between the US and Cuba took place, does that mean that US citizens can go to Cuba freely and vice versa? Show that if m/n is a good approximation of $\sqrt{2}$ then $(m+2n)/(m+n)$ is better What should I do if my Windows computer seems to be infected with a virus or malware?  What are the symptoms of an infection? What should I do after noticing an infection? What can I do to get rid of it? how to prevent from infection by malware?   This question comes up frequently, and the suggested solutions are usually the same. This community wiki is an attempt to serve as the definitive, most comprehensive answer possible. Feel free to add your contributions via edits. How to find roots of $X^5 - 1$? How to find roots of $X^5 - 1$? (Or any polynomial of that form where $X$ has an odd power.) Using regex inside if clause in bash Look at this if block:  #!/bin/bash  str="m.m" if [[ "${str}" =~ "m\.m" ]]; then     echo "matched" else     echo "not matched"     exit 1 fi  exit 0   This should print "matched", but it doesn't. Where am I going wrong? How can I prevent data theft from a company?  Are there any solutions to this problem?  Is there any software that encrypts important data (.doc, .xls, etc.) and prevents it from copying?  I need a solution where employees can read/write documents but can not copy then to a USB drive or e-mail them. Problem  Let $n \geq 3$ be an odd number and let $G=\{1,...,2n\}$ be a group of order $2n$. Let $\phi:G \to S_{2n}$ be the morphism defined by $\phi(g_i)(g_j)=g_ig_j$ and let $H=\phi^{-1}(A_{2n})$. Show that $H$ is a proper normal subgroup of $G$.  So basically the result of this problem is that every group of order $2n$, with $n$ odd, is not simple.   The morphism $\phi$ can be seen as the action of $G$ on itself by left multiplication. Since $A_{2n} \lhd S_{2n}$, then $H=\phi^{-1}(A_{2n}) \lhd G$. I am stuck at showing that $H \neq \{1\},G$. All I could think of is that $G$ has an element $g$ of order $2$, then $\phi(g)$ is of order $1$ or $2$, but it is easy to see that $\phi(g)=1$ if and only if $g=1$, so $ord(\phi(g))=2$. I don't know which are the elements of order two in $S_{2n}$. I would appreciate some help to solve the problem, thanks in advance. Extract substring in Bash Given a filename in the form someletters_12345_moreleters.ext, I want to extract the 5 digits and put them into a variable.  So to emphasize the point, I have a filename with x number of characters then a five digit sequence surrounded by a single underscore on either side then another set of x number of characters.  I want to take the 5 digit number and put that into a variable.  I am very interested in the number of different ways that this can be accomplished. 3D bodies in TikZ Is there like a library that has default geometric figures like the ones shown below. Am not acquainted yet with 3D diagrams in TikZ. How to skip validation when a specific button is clicked? How do I find a command if I don't know its name? Downgrading from OS X 10.9.2 to 10.9 Getting / setting file owner in C# How to order files by last modified time in ruby?