Prevent duplicate Insertion of records?
Display error message if same record exist with combination of some fields?
How to calculate polygon areas and perimeters using QGIS? I have been looking at the plugin API a bit more in depth recently and I was wondering what real differences there were between action and filter hooks. They both are events that receive data as a parameter and they seem to both be able to do the same things.  Obviously I see that actions are called when actions take place and filters are called when data is manipulated, but it seems to just be a semantic naming difference.  Besides semantics and what they are used for, what real differences are there between them? Question :   Determine all natural numbers n such that 3   divides $n\cdot2^{n}+1$  Actually I don't have any ideas to approach but  my efforts :   I see $n=1,2,7,8,13,14$ so I think :   $n=6k+1$ and $n=6k+2$ $k\in \Bbb{N} $  If I'm not wrong but I don't know how ? I prove it ? Assume f is a bounded continuous function on $\mathbf{R}$ and $X$ is a random variable with distribution $F$.  Assume for all $x \in \mathbf{R}$ that  $$ f(x) = \int_\mathbf{R}f(x+y)F(dy) $$  Please help conclude that $f(x+s) = f(x)$ where $s$ is any value in the support of $F$.  The hints that I have come across are to use Martingale theory and consider $\{ X_n\}$ to be i.i.d. with distribution $F$ and make a martingale with some function of $S_n = \sum_{j=1}^nX_j$.    Thanks! In QGIS, I want to make polygons from a set of intersecting lines. Looking at the screenshot here, I want chop the purple line (it's a closed line, not a polygon) where it crosses the green and blue lines. The result hopefully would be a polygon where I have hatched in red. I have quite a few to do.    I have tried the QGIS tool previously called Polygonizer (see ), but now called Polygonize. This is a tool that, given a layer containing a set of crossing lines arranged essentially like a # symbol, should create a polygon from the enclosed space in the middle.  (In QGIS 2.01, Polygonize is in the top menu bar: go to Processing -> Commander, then type "Polygonize" on the command line, select "Processing algorithm: Polygonize" from the list that appears, and press Enter.)   At the momemt Polygonize doesn't work properly. Is there another way to do what I want to do?   For example, wouldn't it be great if the QGIS 'Split Feature' command was able to work like MapInfo's 'Polyline Split', where you select the polygons to split, then select an already-drawn polyline to split by, and voila. But Split Feature seems not to work that way.    UPDATE: Detail of problems with running Polygonize.  I have 64-bit QGIS 2.01 installed to Windows 7 from the installer package, QGIS-OSGeo4W-2.0.1-3-Setup-x86_64.exe.  The routine processed an output from my shapefile, but did not create a polygon from the region enclosed by intersections between the blue, green and purple lines. (Note: in the actual input file, all these lines are on the same layer; the screenshot just colours them differently for illustrative purposes). What I got instead was a simple conversion of the purple outline to a polygon, as though the program completely ignored the blue and green lines.  When I node-edited the closed purple line, to open it so that I had a set of open crossing lines like in the # symbol, and then ran Polygonize again, nothing happened. No crash, but no visible polygon in the output file.    When I used Nick Hopton's own  data, I got a crash; see screenshot: If $ \langle G, \star \rangle $ is an abelian group, then for all $a, b \in G$, show that $(a \star b)^{n} = a^{n} \star b^{n}$. If $ \langle G, \star \rangle $ is an abelian group, then for all $a, b \in G$, show that $(a \star b)^{n} = a^{n} \star b^{n}$.  I am stuck at the first step, unable to figure out how to start. I am just beginning on group theory, and know that an abelian group obeys commutativity, associativity, has an identity element, and each element is invertible. I am unable to figure out how to apply this for the proof. Any hints on how I should get started? In The Dark Knight Rises, at the beginning of the film Bane kidnapped a guy from the plane, then he drew blood from the guy he kidnapped. And then throughout the movie I didn't see any news regarding this kidnap scene. But, before the end of the movie we find out that the guy kidnapped by Bane was Dr Pavel. Bane needed him to convert the fusion project into a nuclear bomb and Dr Pavel was the only guy who could have done it.   But I don't understand, why did he draw blood from Dr Pavel? MiKTeX: Two things did not succeed I installed a new hard drive on my computer this summer and I'm trying to install MiKTeX on it now. There were no problems when I was using my old hard drive. My OS is Windows 8.1. I'm trying to compile the following very basic file with pdfLaTeX:  \documentclass{article} \begin{document}   hi   \end{document}   Here's the first error I get:  Sorry, but "MiKTeX Configuration Utility" did not succeed.     The log file hopefully contains the information to get MiKTeX going again:   C:/Users/Tyler/AppData/Local/MiKTeX/2.9/miktex/log/initexmf.log  You may want to visit the MiKTeX project page (http://miktex.org), if you need help.   With the following errors in the log:  2015-09-03 22:23:51,625-0400 INFO  initexmf - starting: MiKTeX Configuration Utility 2.9.5700 (MiKTeX 2.9 64-bit)    2015-09-03 22:23:51,631-0400 FATAL initexmf - Windows API error 32: The process cannot access the file because it is being used by another process. 2015-09-03 22:23:51,631-0400 FATAL initexmf - Info: C:\Users\Tyler\AppData\Local\MiKTeX\2.9\miktex/data/le\1e0c0e04cb3be262fbb8e6e7ace14b67.fndb 2015-09-03 22:23:51,631-0400 FATAL initexmf - Source: Libraries\MiKTeX\Core\win\winFile.cpp 2015-09-03 22:23:51,631-0400 FATAL initexmf - Line: 520   The second error:  Sorry, but C:\Program Files\MiKTeX 2.9\miktex\bin\x64\miktex-pdftex.exe did not succeed. The log file hopefully contains the information to get MiKTeX going again:   C:/Users/Tyler/AppData/Local/MiKTeX/2.9/miktex/log/miktex-pdftex.log  You may want to visit the MiKTeX project page (http://miktex.org), if you need help.   with the following errors in the log:  2015-09-03 22:16:18,715-0400 INFO  miktex-pdftex - running 'initexmf --quiet --update-fndb' to refresh the file name database 2015-09-03 22:16:18,746-0400 FATAL miktex-pdftex - The operation failed for some reason. 2015-09-03 22:16:18,746-0400 FATAL miktex-pdftex - Info: C:\Program Files\MiKTeX 2.9\miktex\bin\x64\initexmf.exe 2015-09-03 22:16:18,746-0400 FATAL miktex-pdftex - Source: Libraries\MiKTeX\Core\Process.cpp 2015-09-03 22:16:18,746-0400 FATAL miktex-pdftex - Line: 290   Things I've tried:   Uninstalling and reinstalling MiKTeX Downloading all packages whose name starts with miktex (which is a solution that worked for someone else in a different thread) Having nothing open but TeXworks when trying to compile Looking for processes that may be related to MiKTeX in the task manager (I didn't find any)   How can I stop these errors? I apologize if my formatting is bad; this is my first stackexchange question. Thank you for your help. I am trying to encrypt an entire USB drive with Truecrypt 7.1a. But as long as there is a partition it will only encrypt that. This will make it appear like the USB is full instead of completely empty, which isnt what Im aiming for.   I've tried with DISKPART, USB Partition Tool, and the built-in storage manager. When I delete all partitions, Windows still assigns the drive a letter and assumes that it has a partition covering the whole space.  I know that it is possible as I have done it with another USB, but I just don't remember how I did it.  Ideas? I've heard that Lubuntu consumes less resources than Ubuntu. Is it because of the fact that Lubuntu uses different desktop environment and window manager?  If so, would it be equivalent to install lubuntu-desktop on top of Ubuntu rather than doing a fresh install? How to remove/hide the recycling bin and All site content from the quick launch or remove the quick launch altogether? Ho can I remove the recycling bin and the 'All Site Content' link from the quick launch or remove the quick launch altogether Limit point of the set $G= \{1/n: n \in \mathbb N\}$? In $\mathbb{R}$ , what is the limit point of the set $G= \{1/n: n \in \mathbb N\}$?  Definition:  A point $p \in X$ is a limit of of E if every neighborhood of $p $ contains a point $q \in E$ $q\neq p$  I do understand $0$ is a limit point but shouldn't it be the case that all the points in [0,1] are limit points by the definition? Vertex count appears incorrect after removing doubles I've only just started studying the Blender Python API. I'm writing a script (see below) to automate something I've done by hand : joining two adjacent cubes and cleaning up the resulting mesh.   After joining and removing doubles, I expect to have 12 vertices instead of 16 in the resulting object. In the Info header, this seems to be the case, but when I print to the windows terminal I still see 16. Also I do see the output from remove_double(), here's a screenshot :    So what's wrong here ? What is the right way to access an object's mesh, and count its vertices ?  Joao  import bpy  # Create two adjacent cubes and join them bpy.ops.mesh.primitive_cube_add(location=(1,0,0)) ob = bpy.context.object bpy.ops.mesh.primitive_cube_add(location=(3,0,0)) ob.select = True bpy.ops.object.join()  # Expect 16 vertices, 24 edges, 12 faces result = bpy.context.object print(str(len(result.data.vertices)))  # Go to edit mode, remove double vertices bpy.ops.object.mode_set(mode = 'EDIT') bpy.ops.mesh.remove_doubles()  # Expect 12 vertices, 20 edges, 11 faces result = bpy.context.object print(str(len(result.data.vertices))) I want to plot the function 1/(x²+y²), but since it tends to infinity in (0,0), I get a horrible plot:  \documentclass{article} \usepackage{tikz} \usepackage{pgfplots} \begin{document} \begin{tikzpicture}   \begin{axis}[grid=both]     \addplot3 [surf] {1 / (x^2 + y^2)};   \end{axis} \end{tikzpicture} \end{document}     By restricting the domain of z, I get anyway a not so nice plot:  \documentclass{article} \usepackage{tikz} \usepackage{pgfplots} \begin{document} \begin{tikzpicture}   \begin{axis}[grid=both,restrict z to domain=0:1]     \addplot3 [surf] {1 / (x^2 + y^2)};   \end{axis} \end{tikzpicture} \end{document}     I would like to get something smooth, which finish in a circle (the circle of the intersection of this function with the plane z=1 for example).  Something like this would be great: How to extend the text under summation symbol without making extra space Add Word Limit to Posts I was reading the CernCourier, my favorite source of message on Higgs and friends. I was rather shocked, when I saw this:     "The mechanism by which neutrino mass is generated is not known."   What? Not known?  says: In Higgs-based theories, the property of 'mass' is a manifestation of potential energy transferred to particles when they interact ("couple") with the Higgs field, which had contained that mass in the form of energy.  Does this mean that neutrinos don't couple to the Higgs field? A person who cannot understand that there may be other perspectives Is the tabu package obsolete? Chat requirement to post images should be made explicit There is (apparently) a 100 reputation requirement to upload and post images through chat.  There isn't any easily located documentation on this, however. 100 global reputation is listed as the privilege for , while the chat FAQ and help both do not mention any restrictions on uploading images.  As such, could somewhere other than  be used to indicate that there is a reputation requirement in chat for uploading images?