How to enable key repeat on Macbook Pro with Yosemite
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I have been reading through my notes and I came across this example and I found it hard to understand so I need some help in explaining how the inverse of this is found.  The set $\mathbb Q(\sqrt 2)=\{a+b\sqrt2$: $a,b$ in $\mathbb Q \}$ is a field. The inverse of an element $a+b\sqrt 2$ in $ \mathbb Q(\sqrt 2$) is  $$\frac{1}{a+b\sqrt{2}}=\frac{a}{a^2-2b^2}+\frac{-b}{a^2-2b^2}\sqrt{2} $$  Can anyone explain  to how the inverse was found? Appreciate your help. OpenVPN restriction Ive set up an OpenVPN server at my place and handed out a certificate to a client who has an openvpn client at his place.  I need require that he should be able to connect to the vpn server only from one IP address (his current one).  How can I do this? Array of zero length Upvoting a downvoted answer will do +2 upvote? I have first downvoted an answer. Then, I decided to leave it the way it was before, meaning to undo my downvote just like it never existed, so I upvote to remove my downvote. Suprise! SO seems to consider it as two upvotes!  Let's say I have an answer with -3, then I decide to downvote it making it -4. After deeper thoughts, I prefer to remove my downvote to let it the way it was before I downvoted, meaning -3, but SO makes it -2 automatically!     Wouldn't it be supposed -3, and not -2? If someone else would have upvoted, it would have made it an upvote +1, not a +2!   Am I missing something here?  In fact, is there a way to tell SO that I only wish to undo my downvote, and not upvote the answer? I may understand why it does behave this way. I only wish to undo my downvote, though. After removing any part the rest can be split evenly. Consequences? Let $S$ be a finite collection of real numbers (not necessarily distinct). If any element of $S$ is removed then the remaining real numbers can be divided into two collections with same size and same sum ; then is it true that all elements of $S$ are equal ?  (I know the result is true if the &quot;reals&quot; are replaced by &quot;integers&quot;.) Greatest common divisor of two positive integers It would be nice to allow users to mark a change in an answer (or question) as minor (e.g. for a simple spelling or grammar correction). A change marked as minor wouldn't push the question on top of the list of questions on the home page or in the feeds. It wouldn't trigger any notification. This could be implemented with a "Minor change" checkbox when editing an entry (a la Confluence). What do you think? Merging banking and casual style in moderncv (2) I really like the solution given in  but what to do if I want to keep the footer information of the casual style from page 2 on? That is, with this solution, I can put the header as I want in the first page, but the footer disappears from the rest of the pages and I do not want that.  I want the header of the first page to look like this (with an empty footer):   And the footer from the second page on to look like this: Concatenating two std::vectors Add space before and after tabular environment The tabular environment does not add any vertical space before or after it. This looks odd, in my view. What is the "correct" or "best" way to add some space? Also, why is there more space after the tabular environment than before it, as seen in the example below?  \documentclass{article} \begin{document} \noindent Here is a line with some text in it.  \begin{tabular}{*4{l}}     Worg &amp; Worg &amp; Worg &amp; Worg\\     Worg &amp; Worg &amp; Worg &amp; Worg\\ \end{tabular}  \noindent Here is another line with some text in it. \end{document} Why are there so many different ways to measure disk usage? How do I disable the screensaver/lock? When I'm watching a film in Mythtv the screen turns to black every 10 - 15 mins and I have to log back into Ubuntu. Very annoying!  How do I disable the black screen / screensaver / logout in Unity?  There no longer seems to be any options to turn the screen saver off as there were in Ubuntu prior to Unity. A book I once owned and read back in late-80's or early-90's that I have not been able to identify title &amp; author for:  It was a SF story concerning tiny native lifeforms, living on either a planetoid or possibly a dead/neutron star, that evolve at a seemingly accelerated time rate. Most of their story is told in the few days of observation (from orbit) by a human expedition to the planet, during which time-frame the race below progresses from stone-age tech level to being more advanced than the human observers, and eventually flying up to meet the humans in small advanced craft powered by micro-singularities... one of which accidentally 'disintegrates' the tip of a human scientist's nose in the process. I had a Windows 7 installation on my 1TB SSD laptop with 4 primary partitions. I wanted to install Lubuntu Linux but the process failed with an error which kind of said:     cannot write MBR   It gave me an option to put an MBR somewhere else or to boot without MBR. None of these options worked and it became a useless install. After this failed dual boot attempt, Windows would give a BSOD wit Code 007 ending - translated to inaccessible boot device. That is a huge NTFS partition and I don't think I have any other media to back it up before formatting it.  My only option is somehow rebuilding the old MBR. Hence I started creating a whole bunch of live USB boot environments  including Super GRUB Disk and MiniTool Partition Editor Live CD, but none of this could ever help me recover my lost MBR.  Win PE 4.0 reported the partition as bad disc and the latest version did not even have an option for MBR. I managed to create a live USB with Testdisk Running on Ubuntu. Whatever partition was seen as HPFS on quick  search was written in the MBR and after this the disc is totally unreadable. Earlier I could see my data through Linux and now I can see nothing.  Here are my questions:   When some data could be read (but Windows would not boot) the MBR was shown as dynamic MBR. As far as I know dynamic MBR is a Windows concept and my Linux attempt would not have converted it. While installing Linux I tried to delete 2 primary partitions which I found to be not really useful (I backed up their data first). So I am not sure what created the dynamic MBR.  Did my dual install convert those basic MBR to dynamic MBR? How did that happen? Here is a screenshot of my deep search attempt with TestDisk which is still in progress. I can see 4 NTFS partitions and one of these two system partitions should be the original MBR. When I tried to write both these system partitions (after stopping the deep search momentarily) I got an error like "partition not found". The file system utility showed me that there is no backup sector and that may be the reason why it cannot be read.  What other Resorts do I have, other than doing a plain format and then using some recovery tool to recover all my lost data? I am not even sure I will recover the folder names.  What other options should I be trying to restore my old MBR so Windows can boot?      Update  as of last I managed to create a live USB w/ W7  I used AOMEI Partition assistant 6.0 and on deeper  search I can see the 909 gb partition - it wont let me select it for recovery. I am sorry if my earlier Q was not clear. I referred to restoring an MBR from a partition . What happened earlier  - superficial search revealed all old partitions but NOT this one   and the later partitions showed up on Deep search &amp; I can select Other partitions for recovery but for this one. It wont let me select the partition I want  Here are some more screen shots    So in summary all that happened is   4 primary windows 7 partition -->  wanted to install Linux so I deleted 2 non bootable partitions after backing them up  ended up with BSOD ( inaccessible boot device ) and noticed that some windows partitions showed up as Dynamic MBR  (using Live USB Linux,gparted )  Use standard testdisk procedure to write MBR from the superficial NTFS-HDFS partition.  this is where I screwed up . Hindsight I should have really done a deep search and caught the 901gb partition to write MBR. I am not sure if I can undo this action  after this - life gets worse. I am  on a ventilator ! Now C is unreadable by windows and linux both and testdisk cannot list file ( see screenshots ) and I have been using a series of live USB's with some recovery s/w but no luck. ALL I want is to recreate that 901gb partition and the boot partition and put the MBR on the latter so I can get my windows 7 back again If a Matrix Has Only Zero as an Eigen-Value Then It Is Nilpotent I'm working on a jet in blender. While thinking about the final scene, I wondered if there was a way to create schock diamonds(also known as Mach diamonds, Mach disks, Mach rings, donut tails or thrust diamonds) in blender.  Hopefully somebody can help, I haven't found anything online.      picture from wikipedia  picture from Tom Clancy's H.A.W.X  thanks for any tips.  tags: mach, schock, diamond, jet, exhaust, afterburner, engine, Physical phenomena  edit: added some pictures, Splitting pieces of a mesh into a new object I've just made a set of armor by using a male body mesh model I downloaded and selecting certain faces from the model and duplicating them to create viable armor pieces.   Since the pieces are just duplicated from the same mesh male model, then it's still part of the same object.   How do I separate the armor piece from the model to make the pieces separate? biblatex, authoryear style: In-text citations display first name initials for certain bibliography entries How can I completely remove a objects (e.g. particle system or groups used in particle systems) in blender using python?  The problem is this: I have a script, with which I create a scene (e.g. adding some normal objects and some group of objects with a particle system). For a research study I would like to repeat that that several times, without restarting blender. So I can create a scene, delete everything, e.g. with a script like this:  import bpy scn = bpy.context.scene for obj in scn.objects:     obj.select = True bpy.ops.object.delete()   If I rerun the script to recreate the scene with objects using particle system, the latter is not shown in the scene anymore (only the placeholders of the particle system). Now I'm not sure where exactly the problem lies, but I guess not everything is immediately deleted, running the script above. Only the startup of blender helps (but that's not what I want) or pushing the purge button. The purge button seems to start the command:  bpy.ops.outliner.orphans_purge()   But I can't get this piece of code to run in the console. It says that "the context is incorrect". Is there a way to purge the unused data by python script? Thanks a lot for any help. How to install Ubuntu 18.04 alongside Windows 10 How to install Ubuntu 18.04 alongside Windows 10?  I have tried different process to install Ubuntu 18.04 in my PC in dual boot mode alongside Windows 10. But it turned out that most of the process didn't worked for my PC. My PC's configuration is 500 HDD, 3.5GHz Intel processor, 4GB RAM, 1GB Intel graphics &amp; supports both BIOS &amp; UEFI.