Proving unit with some condition
If an element has a unique right inverse, is it invertible?
Max Block Height for a Redstone Contraption What's the max height of blocks for a redstone contraption to still work even I'm in the surface below?  Like if I build a pillar to the max height of blocks and build a redstone contraption there, is it still going to work even I'm on the surface below? Too many requests? I went to stackexchange.com and clicked on SO.  After a few minutes I got a message saying "We have detected too many requests from this IP.  If this is an error contact us."  That was the second SE page I tried to visit.  Second only to SE.com  I moved to another machine and it's working now.  The first machine usually works, I don't know if a got assigned a blacklisted IP or what.  Anyone know what's going on?  EDIT: I would really rather not post my IP on the web, but if a diamond or dev asks I'll email it.  EDIT 2: I looked at badp's dupe suggestion and can verify that I have no scripts frequently calling SO. Manipulating the axis on a ListLinePlot Can't start mongod , configured storage engine is 'mmapv1' I try to run mongo v2.6.10 on Ubuntu 16.04 lts:  mongod   And get the result:  mongod --help for help and startup options 2017-09-16T12:15:19.660+0430 [initandlisten] MongoDB starting : pid=15959 port=27017 dbpath=/data/db 64-bit host=mahdi-MS-7808 2017-09-16T12:15:19.660+0430 [initandlisten] db version v2.6.10 2017-09-16T12:15:19.660+0430 [initandlisten] git version: nogitversion 2017-09-16T12:15:19.660+0430 [initandlisten] OpenSSL version: OpenSSL 1.0.2g  1 Mar 2016 2017-09-16T12:15:19.660+0430 [initandlisten] build info: Linux lgw01-12 3.19.0-25-generic #26~14.04.1-Ubuntu SMP Fri Jul 24 21:16:20 UTC 2015 x86_64 BOOST_LIB_VERSION=1_58 2017-09-16T12:15:19.660+0430 [initandlisten] allocator: tcmalloc 2017-09-16T12:15:19.660+0430 [initandlisten] options: {} 2017-09-16T12:15:19.660+0430 [initandlisten] exception in initAndListen: 28574 Cannot start server. Detected data files in /data/db created by storage engine 'wiredTiger'. The configured storage engine is 'mmapv1'., terminating 2017-09-16T12:15:19.660+0430 [initandlisten] dbexit:  2017-09-16T12:15:19.660+0430 [initandlisten] shutdown: going to close listening sockets... 2017-09-16T12:15:19.660+0430 [initandlisten] shutdown: going to flush diaglog... 2017-09-16T12:15:19.660+0430 [initandlisten] shutdown: going to close sockets... 2017-09-16T12:15:19.660+0430 [initandlisten] shutdown: waiting for fs preallocator... 2017-09-16T12:15:19.660+0430 [initandlisten] shutdown: lock for final commit... 2017-09-16T12:15:19.660+0430 [initandlisten] shutdown: final commit... 2017-09-16T12:15:19.660+0430 [initandlisten] shutdown: closing all files... 2017-09-16T12:15:19.660+0430 [initandlisten] closeAllFiles() finished 2017-09-16T12:15:19.660+0430 [initandlisten] dbexit: really exiting now   error :     exception in initAndListen: 28574 Cannot start server. Detected data   files in /data/db created by storage engine 'wiredTiger'. The   configured storage engine is 'mmapv1'., terminating   after update mongodb show error . How do I calculate the resistor value for a simple LED circuit? I have a simple circuit:    The max rating (current) of the LED is 30mA.  How can I work out what the resistance of the resistor needs to be? By using ohm's law I found it to be \$3V/0.03A = 100 \Omega\$. However using software called , and trial and error I got the minimum possible resistance to be 36&Omega;. However, if I use a 35&Omega; resistor, then the LED breaks. Is the software wrong, or (more likely) am I doing something wrong? Is an automorphism of the field of real numbers the identity map? Is an automorphism of the field of real numbers $\mathbb{R}$ the identity map? If yes, how can we prove it?  Remark An automorphism of $\mathbb{R}$ may not be continuous. Singular or plural verb with two subjects Possible Duplicate:          I'm writing an interrogative sentence questioning someone else's writing:  "Is grammar and spelling correct?"  Can I use the singular verb "is" or must it be the plural verb "are"?  (The sentence is one of a list of interrogative questions about the quality of writing, e.g. "Are words in the right order?" "Does the copy flow?") Proving that if $A$ is a $8\times 8$ matrix over $\mathbb{R}$ and $A^3=A$, then $A$ is diagonalizable. If $A$ is a $8\times 8$ matrix over $\mathbb{R}$ and $A^3=A$ then prove that $A$ is diagonalizable.   I have got that the minimal polynomial of $A$ may be $x^3-x$ or $x$ or $x(x+1)$ or $x(x-1)$ in the 2nd case it is not possible in the 3rd and 4th cases the matrix will be the identity matrix or identity matrix multiplied by a scalar!  But I cannot reach any further. Where to find cross section data for $e^{-}$ + $p$ $\longrightarrow$  $p$ + $e^{-}$ ?  does not include it. What determines which pokemon you are allowed to place at a gym? So I defeated a Team Valor gym earlier today using my highest cp Pidgeot. It turned grey, so I went to claim it with that Pidgeot, except it wouldn't let me. What determines which pokemon you are allowed to place at a gym? Is it just those who have participated in a battle can't? Is there a time period for it? What about training at a friendly gym? Does this impact anything? Can I travel to Paris for a day with a single entry visa for Schengen from Switzerland? "Timed Out" error when trying to join server Where do I find the power cells to restore the bunker door in “Ancient armory”? I am trying to find the power cells to access the amor. Yet the placement is unmarked and I thought I would eventually stumble over them clearing the cauldrons.  Yet after clearing three cauldrons (RHO, XI, and Sigma) I still only have the one from the very beginning.  Where do I find all the power cells? Homeomorphism that maps given points to given points Problem with defining shortcuts for TikZ matrices For  I wanted to define a shortcut for creating a matrix inside a tikzpicture. As a minimal example, consider  \newcommand\mymatrix[1]{     \begin{tikzpicture}         \matrix[matrix of math nodes] {#1};     \end{tikzpicture}% }   which would then be used as  \mymatrix{     a &amp; b \\     e &amp; f \\ }   But LaTeX complains about Package pgfbasematrix Error: Single ampersand used with wrong catcode at the line with the closing brace of \amatrix{}. Why does it do that and how can I avoid it? Installing Ubuntu after Windows 10 How can I install Ubuntu on Windows 10 as dual boot? I have a DVD with Ubuntu and when I tried to install Ubuntu I had a warning that the hard disk will be erased. How can I install Ubuntu and keep my Windows 10? Thanks Eclipse was working as good as anything on 14.04. I did a clean install of 16.04 and installed Eclipse. But it runs a Java program only once after which it just hangs during subsequent attempts before I remove and reinstall it. I have tried many things, since in the beginning I needed CDT as well. Right now, I just want to remove everything related to eclipse and do a fresh install (remove all the configs and dependencies). Please note that I used Eclipse installer to install Eclipse and the eclipse icon image is also not appearing (the default ubuntu ? is appearing). I'm a bit confused about what Gstreamer does.   Does it come installed by default with Ubuntu ?  If so which version  is the default for 14.04?  Does it provide hardware accelerated video playback (decoding) ?   If so does it work with VA-API and VDPAU or is it an alternative to those?    Thanks for your help I was viewing  question in the Android app. As shown in the below screenshot, while the top shows "4 answers", the bottom button will ask to "View 6 Answers". After checking the question, there are indeed six answers to the aforementioned question.    I am using Android 4.3 with the 0.1.64 version of the app.    EDIT: The changes to the UI made this question obsolete. What is the most elegant way to check if the directory a file is going to be written to exists, and if not, create the directory using Python? Here is what I tried:  import os  file_path = "/my/directory/filename.txt" directory = os.path.dirname(file_path)  try:     os.stat(directory) except:     os.mkdir(directory)         f = file(filename)   Somehow, I missed os.path.exists (thanks kanja, Blair, and Douglas). This is what I have now:  def ensure_dir(file_path):     directory = os.path.dirname(file_path)     if not os.path.exists(directory):         os.makedirs(directory)   Is there a flag for "open", that makes this happen automatically?