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October 4

system font del Capitán

I just noticed that in OS X El Capitan the system font is no longer Helvetica. Anyone know what it is? —Tamfang (talk) 06:29, 4 October 2015 (UTC)

Wikipedia:WHAAOE: San Francisco (2014 typeface) 731Butai (talk) 08:18, 4 October 2015 (UTC)

• Sometimes the tricky part is knowing where to look. — I wonder why it doesn't show up (at least by that name) in Font Book. —Tamfang (talk) 04:54, 5 October 2015 (UTC)

...You aren't going to believe this, but it doesn't show up because it's not installed. (See my earlier response, below, for detailed information about how this confusing statement works in practice). If you wish to install this font for use in anything-other-than-the-built-in-system-UI, you need to download it: the main Fonts page for developers links to San Francisco for iOS; and for more information regarding font types on iOS and OS X; and you need to sign a developer licensing agreement.

Introducing the New System Fonts, from WWDC 2015. Nimur (talk) 01:04, 5 October 2015 (UTC)

Grover's Algorithm

In the atricle about Grover's algorithm in the second (and last) figure : why after performing $U_{\omega }$ on $|s\rangle$, the result is not at the left of $|\omega \rangle$ as for a reflection around $|\omega \rangle$? 80.246.136.92 (talk) 09:40, 4 October 2015 (UTC)

For whatever reason, the definitions of U_ω and U_s differ by a sign. U_ω is a reflection through the plane normal to ω, while U_s is a reflection through all axes perpendicular to s (or the negative of a reflection through the normal plane). See the first two formulas in Grover's algorithm#The first iteration. -- BenRG (talk) 00:05, 5 October 2015 (UTC)

I'd thought that the reflection is around $|\omega \rangle$. Thank you! 185.32.179.149 (talk) 17:43, 5 October 2015 (UTC)

Trace route

when u do a trace route u see many different companies in the hops. can they see your internet traffic? 62.37.237.15 (talk) 10:53, 4 October 2015 (UTC)

Yes, that's how the internet works. Usually the traffic goes from your ISP to their upstream provider, often to a national provider, through a Internet exchange point, over an international connection, through another IXP, and back through another chain of suppliers. Yes, they can all see your traffic, and so can governments which force them to provide the government with a copy of the traffic. That's why encryption on the internet is so important. 2.126.122.210 (talk) 12:41, 4 October 2015 (UTC)

The usual analogy given is that unencrypted traffic is like a postcard. Anyone handling it can look at what you're sending without your knowledge. --71.119.131.184 (talk) 07:38, 5 October 2015 (UTC)

Finding Unitary transformation

I am looking for some Unitary transformation that satisfies $T(\alpha |0xz\rangle +\beta |1yz\rangle )=|1yz'\rangle$ where $|x\rangle ,|y\rangle \in \{|0\rangle ,|1\rangle \},|z\rangle =|000...\rangle ,\alpha ^{2}+\beta ^{2}=1$ ($|z\rangle$ is actually grabage for storing another data, if needed), and I can't find one... 31.154.92.179 (talk) 13:16, 4 October 2015 (UTC)

I think this is impossible, though it's not obvious to me how to prove it. T is linear, so, for example, $|000\cdots \rangle$ needs to map to a constant output independent of which vector starting with $|1\rangle$ you've added it to, but you seem to need it to map to different things depending on that other vector. -- BenRG (talk) 17:00, 5 October 2015 (UTC)

Why it's a constant? $T(|000...\rangle +|x\rangle )=T(|000...\rangle )+T(|x\rangle )\neq T(|000...\rangle )+T(|y\rangle )=T(|000...\rangle +|y\rangle )$, so the result depends on the other vector (whether it's $|x\rangle$ or $|y\rangle$) too 185.32.179.149 (talk) 17:41, 5 October 2015 (UTC)

Assume also that $\alpha ,\beta$ are fixed, and $\alpha \approx 1,\beta \approx 0$. Then, what Unitary transformation does this? 80.246.136.9 (talk) 05:47, 6 October 2015 (UTC)

Here's a proof: unitary transformations preserve inner products, so we have

$({\bar {\alpha }}\langle 0xz|+{\bar {\beta }}\langle 10z|)(\alpha |0xz\rangle +\beta |11z\rangle )=\langle 10z'|11z''\rangle$

which reduces to |α|² = 0. So there is no solution when α ≠ 0. (There are solutions when α = 0, such as the identity function if β = 1.) -- BenRG (talk) 07:22, 6 October 2015 (UTC)

It's a beatuiful proof! Thank you! 31.154.92.179 (talk) 15:44, 6 October 2015 (UTC)

Searching for terms and specifics on FB groups

Some of the FB groups I'm a member of have huge amount of posts daily. However, a lot of it is reposts and rubbish that I'm not interested in.

Is there any way to search for what I want based on terms, as occasionally buried beneath the clutter there are some gems.

From what I can tell, the way the group works is as you scroll down it loads additional portions of content. So just using the search facility in my browser seems to be inadequate without having to spend a lot of time scrolling down, waiting for sections to load which again isn't very efficient. — Preceding unsigned comment added by 80.195.27.47 (talk) 16:38, 4 October 2015 (UTC)

Most of the groups I belong to, somewhere near the top of the group's main page there is a magnifying glass search bar that lets you search for members of the group or content in the posts to the group. RegistryKey^(RegEdit) 07:48, 5 October 2015 (UTC)

Semi-transparent window in legacy RHEL/CentOS?

Is there any evidence that the X server in CentOS 4 (or Red Hat Enterprise Linux 4) really supports compositing? After I added the following settings to /etc/X11/xorg.conf:

Section "Extensions"
        Option          "Composite"     "Enable"
EndSection

I noticed that the composite extension appears in the output of xdpyinfo:

[root@localhost tmp]# xdpyinfo
    .            .
    .            .
    .            .
number of extensions:    31
    BIG-REQUESTS
    Composite
    DAMAGE
    DOUBLE-BUFFER
    DPMS
    .            .
    .            .
    .            .

We want to adjust the window opacity on-the-fly in our Qt application. It seems that enabling compositing is the way to go (otherwise we may need to emulate the transparency effect by contents propagation). But weird artifacts appears after composite extension is turned on. Is it because xdpyinfo reports incorrectly or something?

Version info. in the system:

CentOS: 4.8

Xorg: 6.8.2

metacity window manager: 2.8.6

Migrating to newer CentOS version should resolve the problem, but our application is too complicated to do the migration easily in short time. - Justin545 (talk) 17:17, 4 October 2015 (UTC)

nul device that remembers

Is there a feature like the ">nul" device that writes data to a non-existent place, except instead of discarding the data immediately it keeps say 10 items in RAM and removes the 10th item every time a new item is written to it. The 10 items can be accessed like normal files. The number 10 is an arbitrary number, it could be 1 or 1000000 doesn't matter. Thanks for your help. — Preceding unsigned comment added by 109.251.203.41 (talk) 20:45, 4 October 2015 (UTC)

Somewhat similar in concept, in Unix there is a /tmp directory where temporary data was written, and a daemon was run each night to erase it all (where I used to work). StuRat (talk) 21:07, 4 October 2015 (UTC)

Your question is not well defined. When you say "is there a feature like", do you mean anywhere in the entire history of computing? Do you mean in a specific operating system or program or programming language? Many programs have a command history or an undo history, many operating systems have a clipboard or scratch disk, they're all "sort of" a place that hold data written to them. Vespine (talk) 02:51, 5 October 2015 (UTC)

Named pipe is also somewhat similar to what you want, but it doesn't have a limit to the number of items written. The program tail can show you the last 10 (or whatever number you specify) lines in a file. We may be able to give you better help if you tell us more about what you want to use such a device for.-gadfium 03:05, 5 October 2015 (UTC)

That would not be a 'nul' device, because a null device by definition does nothing to data transmitted to it. It does not copy, does not store, does not transmit it anywhere – it just ignores anything you put there.

IMHO a device you described may get called a 'limited queue' or better a 'tail queue' or a 'cyclic buffer' device. I don't know of such device built-in into any operating system, but see gadfium's reply above for a Unix/Linux-like command-line tool example. --CiaPan (talk) 14:35, 5 October 2015 (UTC)

I can see that there might be a use for a modified version of nul that logs instead of ignores. It could help in troubleshooting certain situations. The question is, does such a modified nul device exist for Linux or BSD?? Is there some sort of logging utility that does the same basic function in Windows?

The question of preventing a log from growing too large by discarding the oldest entries seems like a separate problem, and one that already has multiple solutions. --Guy Macon (talk) 15:25, 5 October 2015 (UTC)

On Unix, to save output, you just redirect it to a file. my_command > my_file is routine. To throw away the output, you just point it at /dev/null instead. If you want to do fancier things, you can use a pipe or socket that's read by another process. So, there isn't any real need for a "modified nul device" on Unix, since things are really the other way around: /dev/null is just a special file. One of the big ideas in Unix is "everything is a file". Regarding the original question, I suspect we might have an X-Y problem here, so I'm going to refrain from trying to give a broad answer. Can the original poster give specific details of what they're trying to do? --71.119.131.184 (talk) 18:57, 5 October 2015 (UTC)

Would that capture everything sent to nul by any part of Linux or BSD? I already know how to make any program I choose send output wherever I choose, but is it possible (for troubleshooting purposes only) to see everything going to nul no matter what obscure library or device driver sends it? The key here is seeing things that you didn't know were being sent to nul. --Guy Macon (talk) 04:25, 6 October 2015 (UTC)

I think there is a way to capture the null-device data. Just remove the special file /dev/null entry from the filesystem and replace it with a normal file. Then it will capture all the output from all processes in the system, that normally goes to a sink. ;) You might also try to rebuild the system with your own null–device driver.
However, if you want to capture just the null'ed output of a single process, I have no idea how you can achieve that. I imagine you would have to replace a system driver for a null device and build-in some command to switch its operation mode (say to switch logging on and off, or to define process ID of the process, whose output should get logged)... --CiaPan (talk) 12:08, 6 October 2015 (UTC)

Ideally you just stop the process from ever opening /dev/null in the first place. You're going about things backwards. If something (like a shell script) is starting the process and pointing it at /dev/null, change that. If some configuration file is telling it to write to /dev/null, change that. If the program is hard-coded to write to /dev/null, and you have the source, change that and rebuild it (and maybe complain to the author(s)). If none of those are feasible, you can start the process in a chroot jail, container, or whatever and ensure /dev/null is a regular file inside it, or start it under the control of a debugger and intercept calls to open(/dev/null, .... --71.119.131.184 (talk) 12:34, 6 October 2015 (UTC)

It occurred to me if you're not familiar with Unix, you might not understand what I'm getting at above. /dev/null is never open by default on Unix. The only default file descriptors (fd for short) on Unix are stdin, stdout, and stderr. So if a process is writing to /dev/null, it either inherited from its parent a file descriptor pointing at /dev/null, or opened /dev/null itself. To illustrate, when you run foo > /dev/null in a shell, the shell fork()s a child, and the child redirects stdout to /dev/null before exec()ing "foo". This is a really good primer on how processes on Unix work. We've also got a number of articles on that discuss Unix: fork-exec might be a good starting point. --71.119.131.184 (talk) 14:14, 6 October 2015 (UTC)

Certainly anyone who is moderately able to write C code could write such a thing for UNIX/Linux using a named pipe. The pipe would feed a daemon of some kind that would read from the pipe, and maintain the "most recent N items" someplace on disk. Most of the time, the daemon would be blocked waiting on it's read request, so it would only consume resources when there data is actually being written to the pipe. If you place the pipe in /dev/notQuiteNull (or whatever) then it would then "just work". It's probably something that could be written in a few dozen lines of code. Start the daemon when the operating system boots up. There might be issues if multiple processes tried to write to your named pipe at the same time, but I'm sure those can be resolved. SteveBaker (talk) 18:43, 5 October 2015 (UTC)

October 5

Apple vs Microsoft

So I installed El Capitan, as one does, and now my Microsoft Natural keyboard is no longer recognized as such. In particular, I no can haz right Command key, which really cramps my style. Shall I wait for a fix, or try to revert? (And how can I reinstall 10.10 without a disk?) —Tamfang (talk) 04:59, 5 October 2015 (UTC)

At the very least, a formal bug report to both Microsoft and Apple is in order.

Regarding a downgrade: I believe this is possible - but not recommended - the procedure is explained on About Recovery; although the recommendation is always to use the newest compatible version.

Nimur (talk) 15:06, 5 October 2015 (UTC)

Did you try the Microsoft Natural Keyboard driver for Mac? It is at [ https://www.microsoft.com/hardware/en-us/d/natural-ergonomic-keyboard-4000 ]. --Guy Macon (talk) 15:31, 5 October 2015 (UTC)

• I already have a Microsoft [peripherals] panel in System Preferences; is this something else? —Tamfang (talk) 16:00, 5 October 2015 (UTC)

More info here: [ http://david.rothlis.net/keyboards/microsoft_natural_osx/ ]. --Guy Macon (talk) 15:34, 5 October 2015 (UTC)

• I took (the first parts of) that advice; at least I have two Command keys again, which was the most urgent thing. —Tamfang (talk) 06:49, 6 October 2015 (UTC)

Could someone with Apple experience answer the above? I am 70% *nix and 30% Windows. --Guy Macon (talk) 18:59, 5 October 2015 (UTC)

Politics of character encoding

Who sets character encoding standards? UTF-8 seems to be the norm now, but who has the last word about changes, who is heard when developing the standard, and how could it incorporate new features?--Bickeyboard (talk) 10:36, 5 October 2015 (UTC)

Nobody has the last word. Go a ahead and use EBCDIC or Baudot code if you want. Of course you have to convince whoever is on the other end to accept your encoding, and they probably already accept UTF-8. As for adding new features, see See Character encoding#History and UTF-8#History. --Guy Macon (talk) 12:24, 5 October 2015 (UTC)

I interpret the question as more along the lines of "Who makes the official decisions on aspects of UTF-8?" The answer to that question is the Unicode Consortium, which is in charge of the Unicode standards. --71.119.131.184 (talk) 18:39, 5 October 2015 (UTC)

The Unicode Technical Committee develops and maintains the Unicode Consortium's standards. Their meetings are open to the public, but only members can participate, and only higher membership levels (starting at $7500/year) can vote. Voting members include Adobe, Apple, Microsoft, and other big companies that care about character encoding standards. -- BenRG (talk) 00:06, 6 October 2015 (UTC)

October 6

Terms for abuse of OO?

I have encountered some forms which think are abuse of OO (Object-Oriented programming) at work. I can't reveal trade secrets, so here is an abstraction.

I have encountered something like this:

public void DrawShape(Shape shape) {
  if (shape is Circle) {
    DrawCircle(((Circle)shape).Origin, ((Circle)shape).Radius);
  }
  if (shape is Rectangle) {
    DrawRectangle(((Rectangle)shape).TopLeft, ((Rectangle)shape).BottomRight);
  }

instead of having the DrawShape method implemented in the class Shape itself as an abstract method, with Circle and Rectangle providing their own implementations.

And I had myself made a method to write the rendered HTML code of a web page control into a string, and another coder found out that it was not working properly for one of his controls. So, instead of writing his own code, or making a subclass of the class I'd written, or introducing a delegate to perform custom rendering, he had hacked my code so that if it encountered a specific class of control, it ran his own, specific code hard-coded to handle a single specific instance. In my opinion, this introduced hard-coded special cases to code intended as fully generic, which I consider abuse.

Are there any specific terms for this kind of programming? JIP | Talk 19:21, 6 October 2015 (UTC)

Perhaps Anti-pattern. Your title and "abuse" reminded me of Abuse of notation in math, but that is rather different -- often tolerated and sometimes even preferred. SemanticMantis (talk) 19:55, 6 October 2015 (UTC)

You'll find plenty of similar examples if you just search for things like /OO(P) abuse/, like this blog post [1]. SemanticMantis (talk) 20:01, 6 October 2015 (UTC)

According to Steve McConnell, this is called "logical coupling", but we don't have an article on the specific topic. Tevildo (talk) 20:23, 6 October 2015 (UTC)

I reformatted your code: the leading spaces are sufficient without <code>, and they mix badly. --Tardis (talk) 13:39, 7 October 2015 (UTC)

It's called "programming in such a way as to make the maintenance programmers hate you". But I made a good career out of cleaning up after such terrible programming practices. I had a t-shirt made saying "Because they're idiots." --jpgordon^{::==( o )} 16:56, 7 October 2015 (UTC)

Installing Linux without ever touching Windows

After seeing a Facebook meme about requiring IE to install other browsers, I got to thinking. Even though I have to use Windows at work, I have only ever used Linux at home for the past decade. I can't even remember how I first installed it, but every subsequent installation has been done using Linux only.

Is it possible to buy a fresh new computer, either as a pre-packaged commercial product or as parts you assemble yourself, and install Linux on it, without ever running Windows on it? Would it require a pre-packaged installation medium of Linux also bought from a store? JIP | Talk 19:29, 6 October 2015 (UTC)

Well, you can't download it off the Internet without some minimal O/S already installed to allow for Internet access. You could install it from a flash drive, if the BIOS was set to boot from a flash drive. Of course, getting the Linux O/S on a flash drive would either require that you pay for it or use another PC to download it. I suppose you could also have a flash drive with just enough O/S on it to enable you to download the rest online. StuRat (talk) 19:35, 6 October 2015 (UTC)

It's hard for me to tell what rules you're operating under. If you're allowed access to some other computer, then you can install Linux without booting Windows on the target PC. Live_USB#Examples_of_Live_USB_operating_systems gives several official Linux distros that can be loaded and booted from a flash drive, also you can make your own - List_of_tools_to_create_Live_USB_systems. Depending on which one you use, that OS can then be used to install other OSs. SemanticMantis (talk) 19:51, 6 October 2015 (UTC)

I didn't have a home machine until 2002 and I wanted it to run Linux. I purchased a PC without an o/s from my neighbourhood PC shop and went to a news ageny and bought a Linux mag with a distro on a "free" CD which was stuck to the cover. I imagine many other people did this too. TrogWoolley (talk) 20:57, 6 October 2015 (UTC)

I met up with some people from my local Linux user group, they will be more than happy to help you install Linux on your computer without even mentioning windows. Vespine (talk) 21:54, 6 October 2015 (UTC)

Concise answers to your questions: yes and no, respectively. Semi-random example: Here are the installation instructions for Debian. --71.119.131.184 (talk) 23:09, 6 October 2015 (UTC)

You can download or buy a Live-CD or a Live-USB which mostly also offer the option of installing the OS. YX-1000A (talk) 00:58, 7 October 2015 (UTC)

Just yesterday, I purchased a new computer. I used my Linux machine to download the latest Centos 7 (Linux) install disk. I turned on the computer and went into the RAID utility to set up the disks (RAID happens long before it gets to Windows). I put the Centos disk in the DVD drive and rebooted after I got RAID set up. It booted up to the Centos install screen, installed Centos, and rebooted into Linux. I never saw Windows. I don't know if it came with Windows or not. How about the computer I used to download Centos? It runs Linux. It came without an operating system. I used my previous computer to download a Linux install disk for it. My previous computer was one of a set of four that I purchased. I had a Redhat disk from the company I work for that I used to put Redhat on each one. My company got the disk from Redhat. So, it wasn't downloaded. How did Redhat get the disk? They likely used their own Redhat computers. You can keep going back and back and back. Eventually, you will hit a time when it was possible to download the original Linux install software using a popular non-Windows computer of the time, such as the Amiga. 209.149.113.94 (talk) 15:57, 7 October 2015 (UTC)

Images not displaying in MS Word

I usually copy WP articles then paste it into my MS Word due to internet security reasons. Recently (from 2nd or 3rd November) it stopped copying the images from the WP articles, just copy's the article's words... How do I need to mitigate this? By the way, I'm using 'Google Chrome' if this helps. -- Space Ghost (talk) 21:00, 6 October 2015 (UTC)

Can you explain the security issue ? One option might be to save the web page as an HTML file, which will also copy the images. You would then use any browser to view it offline. I believe you right-mouse-click on the page, being careful not to be on a pic (or then it will just save that pic), then click Save As. StuRat (talk) 00:30, 7 October 2015 (UTC)

Maybe this helps: http://smallbusiness.chron.com/copy-word-42912.html If it doesn't work, perhaps something happened to your google chrome or MS Word installation (automatic upgrade)? Ssscienccce (talk) 03:49, 7 October 2015 (UTC)

I tried restoring my PC, it doesn't restore. I don't usually restore, I reinstall, I restored it once before, fearing that it will mess up my PC, knowingly, I had to restore because my sound system wasn't working. Now I can't even reinstall because of the Antivirius I possess; keeps me secure from the internet.

StuRat: The security issue, you won't believe it, even if I tell you. WP-user Guy Macon came up with something really good i.e. Tails (operating system) OS. The guy who downloads things for me is giving me attitudes indirectly, probably I've gone to him for downloading purposes a bit too much, so 50/50% chance now that I might download it from him or I might do it myself. The option that you guided above works by the way; its just a long process. Thank you, this has to do for now. -- Space Ghost (talk) 18:56, 7 October 2015 (UTC)

October 7

Keep track of changes on local file (one user)

How can I keep track of changes on a local file (like .emacs, or other config file)? What I am doing now is making copies of it when I change something (in case it does not work, I pull the back-up). Would learning about git make sense or is it overkill? Could this be dealt with a couple of cli commands? --YX-1000A (talk) 00:44, 7 October 2015 (UTC)

If it's a small file, then what you are doing (making "full image backups") is probably best. Where this method becomes problematic is if you have huge files with frequent minor updates. In this case, all those backup copies take up too much space, and you might consider "incremental backups", which only store the changes since the last full image backup. But, if you can avoid the complexity this involves and stick with full image backups alone, all the better. StuRat (talk) 03:56, 7 October 2015 (UTC)

The full-bore solution is to use a version control system like git, yes. It might be overkill for small, simple files, but small, simple files have a tendency to grow into large, complex ones over time. I'm moderately familiar with emacs, and I know that hardcore users tend to develop extremely complex .emacs files over time. Many such users manage their .emacs with a VCS. And if you are doing or intend to start doing serious programming, you'll need to become very familiar with VCS anyway --71.119.131.184 (talk) 04:17, 7 October 2015 (UTC)

emacs does backups, including numbered backups (one for each save.) Using distributed VCS-s like git for config files sounds a bit overkill-y (at home, anyway), but that's not to dissuade you from learning it. There's also the option, as StuRat rightly said, of making daily incremental snapshots of the FS with something like backintime, which uses rsync. Making them onto external storage guards against user error and hardware failure alike and is a bit like crude versioning Asmrulz (talk) 04:26, 7 October 2015 (UTC)

Unitary Transformation

Is there (or, when is there) any unitary transformation that "changes" the probabilities (not only the phases) of the states? That is, $\forall |x\rangle ,|y\rangle \in \{|0\rangle ,|1\rangle \}\ T(\alpha |0xz\rangle +\beta |1yz\rangle )=\alpha '|0xz'\rangle +\beta '|1yz'\rangle$ where $\alpha ,\beta ,\alpha ',\beta '$ are fixed, and $|\alpha |\neq |\alpha '|,|\beta |\neq |\beta '|$ (and not only a phase change, i.e, not only $\alpha \neq \alpha ',\beta \neq \beta '$), and $|\alpha |^{2}+|\beta |^{2}=1,|\alpha '|^{2}+|\beta '|^{2}=1,0\notin \{\alpha ,\alpha ',\beta ,\beta '\},|z\rangle =|00...\rangle$, and maybe $z=z'$. (This question generalizes my prior question)31.154.92.179 (talk) 06:04, 7 October 2015 (UTC)

There is none. Proof: I'll write z'_xy for the z' associated with a particular value of x and y. By linearity,

$T(\alpha |00z\rangle +\beta |10z\rangle +\alpha |01z\rangle +\beta |11z\rangle )=\alpha '|00z'_{00}\rangle +\beta '|10z'_{00}\rangle +\alpha '|01z'_{11}\rangle +\beta '|11z'_{11}\rangle =$

$T(\alpha |00z\rangle +\beta |11z\rangle +\alpha |01z\rangle +\beta |10z\rangle )=\alpha '|00z'_{01}\rangle +\beta '|11z'_{01}\rangle +\alpha '|01z'_{10}\rangle +\beta '|10z'_{10}\rangle$

so $z'_{00}=z'_{01}=z'_{10}=z'_{11}=z'$. Then, much like last time, we have

$({\bar {\alpha }}\langle 0xz|+{\bar {\beta }}\langle 10z|)(\alpha |0xz\rangle +\beta |11z\rangle )=({\bar {\alpha }}'\langle 0xz'|+{\bar {\beta }}'\langle 10z'|)(\alpha '|0xz'\rangle +\beta '|11z'\rangle )$

which reduces to $|\alpha |^{2}=|\alpha '|^{2}$. -- BenRG (talk) 07:34, 7 October 2015 (UTC)

A beautiful proof, again! Thank you very much! 80.246.139.106 (talk) 09:14, 7 October 2015 (UTC)

Unitary Transformation (in the last time, I guess)

Is there any unitary transformation that satisfies $\exists |z\rangle \forall |x\rangle ,|y\rangle \in \{|0\rangle ,|1\rangle \}\exists |z'\rangle \ T(|0xz\rangle +|1yz\rangle )=|0yz'\rangle +|1xz'\rangle$? (i.e, it swaps $|x\rangle$ and $|y\rangle$, and writes some data, $|z'\rangle$, if needed). 31.154.92.179 (talk) 13:49, 7 October 2015 (UTC)

Oppss... Seems that it's only "Not" on the first qubit... Thanx however :) — Preceding unsigned comment added by 31.154.92.179 (talk) 19:23, 7 October 2015 (UTC)

This what I wanted to ask:

Is there some unitary transformation that satisfies $\forall |a\rangle ,|b\rangle ,|c\rangle ,|d\rangle \in \{|0\rangle ,|1\rangle \}\ T(|abz\rangle +|cdz\rangle )=|adz'\rangle +|cbz'\rangle$? (again, it swaps $|b\rangle$ and $|d\rangle$).

I know that no unitary transformation satisfies $\forall |a\rangle ,|b\rangle ,|c\rangle ,|d\rangle \in \{|0\rangle ,|1\rangle \}\ T(|ab\rangle +|cd\rangle )=|ad\rangle +|cb\rangle$, since it's the identity on $|ab\rangle$ and on $|cd\rangle$, and non-identity on their sum - contradicting linearity. But I don't know if when we add $|z\rangle$ at the end, this transformation exists?132.66.201.186 (talk) 09:16, 8 October 2015 (UTC)

I think a similar counterargument still works when z is added: we have $T(2|abz\rangle )=2|abz'_{abab}\rangle$, so, e.g., $T(|01z\rangle +|10z\rangle )=|01z'_{0101}\rangle +|10z'_{1010}\rangle$ which can't equal $|00z'\rangle +|11z'\rangle$ for any $z'$. -- BenRG (talk) 21:29, 8 October 2015 (UTC)

(For the purpose of thinking intuitively about this, it may help to imagine superpositions as classical probability distributions. $|abz\rangle +|cdz\rangle$ is like a probabilistic choice between $|abz\rangle$ and $|cdz\rangle$. The gate "sees" one or the other, not both. This is essentially what linearity means. The gate can't produce output that combines a (or b) with (c or) d because it can't see both of them. I don't think this qualifies as a proof but it may be helpful in understanding what unitary transformations are likely to exist.) -- BenRG (talk) 21:53, 8 October 2015 (UTC)

I'd tried to show that it's always the same $|z'\rangle$, and hadn't succeeded... Thank you for the proof and for your explanation! 31.154.92.193 (talk) 11:07, 9 October 2015 (UTC)

How to make layers translucent in GIMP?

I have a question about GIMP. I want to paste a layer on top of another layer, but I want the pasted layer to be translucent, so that the pixels in the pasted part end up being a half-and-half mix of those of the original layer and those of the pasted layer. In other words, I want the pasted layer to have a 50% opacity instead of 100%. How can I do this? JIP | Talk 18:35, 7 October 2015 (UTC)

In the layer dockable, select the layer you want to change, and (below its mode dropdown) is a layer opacity slider. -- Finlay McWalterᚠTalk 18:53, 7 October 2015 (UTC)

GIMP is not always intuitive but this is might be what your after. >http://www.google.co.uk/url?q=http://www.gimp.org/tutorials/Layer_Masks/&sa=U&ved=0CBQQFjAAahUKEwiJ6O7ohbHIAhXFrD4KHdpSAHE&usg=AFQjCNHaBzSNIcTqK8OCfEVUII75okCoxQ < --Aspro (talk) 19:07, 7 October 2015 (UTC)

Layer masks aren't the right solution here; too complicated. Finlay is right, just adjust the opacity of the floating layer in the Layers dialog, before anchoring it. Looie496 (talk) 19:15, 7 October 2015 (UTC)

BTW, what you describe is 50% transparency, which is not the same as translucency. Translucent materials let the light through from behind, but blur it, as in frosted glass used in some bathroom windows for privacy. StuRat (talk) 04:18, 9 October 2015 (UTC)

HI there, I have on my macbook 2000 contacts on my iphone also, and I would like to know,

how I can sort them, that I will have shown users which have registered there number on FaceTime / apple-ID. Searching every of 2000 accounts each for each by hand is really a hard way. --Hijodetenerife (talk) 20:11, 7 October 2015 (UTC)

What I think OP is asking is: I have 2000 contacts. How can I sort out those who I also have FaceTime contact info for. And if I'm right, you can't. That's according to the three iPhone users in the room with me now. Dismas|^(talk) 00:04, 8 October 2015 (UTC)

I agree that I don't think it's possible with the default iOS contacts management. But OP could look into other apps. Here [2] is a suggestion, and links to a few alternatives, and I think it's likely that one of them will do what OP asks. SemanticMantis (talk) 14:43, 8 October 2015 (UTC)

October 8

Wikipedians who edit Wikidata

Hi, I'm curious to know how many Wikipedia editors also edit Wikidata and vice versa?

I actually asked this question already on the general Reference Desk and was told to ask here. Wavelength perhaps you could give your partial answer here? thanks--Plarishome (talk) 06:14, 8 October 2015 (UTC)

Plarishome, some editors are categorized in Category:Wikipedians who contribute to Wikidata, but I do not know the total number.

—Wavelength (talk) 16:20, 8 October 2015 (UTC)

How common is following the moon for data centers?

How many data centers, cloud computing platforms, or the like, follow the moon in their activities? That is, how many companies distribute their work-load to the night-shift (if they have computing capabilities around the globe, obviously)? Is it feasible at all? It looks like a good idea to save on the electricity bill, but technically things could get tough. Keeping integrity, low latency, security, and predicting load (among other problems) do not seem as a minor task.--YX-1000A (talk) 14:04, 8 October 2015 (UTC)

The moon does not rise at night: the roughly 29-day cycle means that the moon is up during the day about half the time. Here are Sun and Moon rise-time tables from The United States Naval Observatory.

In fact, at the time of this writing, you would not see the moon from Earth at all during most nights until the very early morning, just about at sunrise. Nimur (talk) 14:36, 8 October 2015 (UTC)

You are completely right that moon != night. However, "following the moon" is the expression of choice for this type of work-flow, even if it's a misnomer. It's the complement of "follow-the-sun" which linguistically makes more sense, but I wonder too whether this scheme gets implemented. The technical problems here are also steep.

Anyway, do data centers transfer at night their processing of information to a computing facility on the antipodes? --YX-1000A (talk) 14:45, 8 October 2015 (UTC)

Thanks for explaining. I don't know much about this, but a quick search does seem to indicate that "following the moon" is done by data centers to save energy costs [3] [4] [5]. I think it will be hard to find hard numbers in terms of how many services do this, how they manage the hand-offs, etc, as some of that is private knowledge that could give the company a competitive advantage, and may be guarded as a trade secret. SemanticMantis (talk) 15:54, 8 October 2015 (UTC)

The links indicate that the concept exists. You might be right that it's difficult to find operational details about this though.YX-1000A (talk) 15:59, 8 October 2015 (UTC)

I guess you're aware that it's also mentioned in the article you linked to with a source Follow-the-sun#Follow-the-moon.

Note however, transferring to the antipodes probably won't be the most sensible solution. While it may help with seasonal effects on on daylight, lots of the earths landmass doesn't have antipodes, and even in cases where it does, the connection costs etc will often vary significantly. It would probably make more sense to worry most about longitude, without worrying so much about latitude. In fact, if you did consider latitude, considering seasonal issues, I imagine it's possible the price would actually by higher or lower for most of the day in one place during part of the year, while different in the other.

Also, I think the most sensible solution is more complicated than following the moon (where moon actually means nighttime). For example, if you consider a place like NZ, I'm not sure that prices during daylight at 1pm would on average be cheaper than (generally) nightime at 8pm. Unfortunately I couldn't find a clear spot price graph showing average price variance over time of day after extensive searching. This is of course assuming you are exposted to spot prices, if you're just a small data centre and aren't, then your rates would generally be 7am-11pm and 11pm-7am. Except for one lines company, who's peak, off-peak and night prices would seem to support my view that 8pm may very well be more expensive on average than 1pm [6] (these are the line chargers, but the unit prices are unsurprisingly similar [7]).

Then we probably get to one of the most significant issues namely that when you're comparing widely differing locations, the electricity price may vary enough that even considering spot prices, it's nearly always cheaper or more expensive. (You also have to consider cooling issues, hence why some data centres locate in places like Iceland, where you can rely mostly on natural cooling during most of the time. Then there are people doing unusual things like designing computers to be installed as heaters [8] [9].)

I would note that if you look at the study indirectly linked in our article, it doesn't seem to discuss night or moon at all. It does mention time of day variance, and stuff like 6am, but it doesn't actually seem to be suggesting following the moon makes sense. For starters it seems to be looking at data centres in the US only. Second, it seems to be looking at far more sophisticated systems either based on real time prices or recent historic prices. These would seem to make far more sense than naïvely following the moon, without considering whether it's actually cheaper to do so. (It also seems to highlight another issue namely that for some types of data centres, your expected load is also going to vary including depending in the time of day.)

My guess is that most of those who've actually implemented this, would have tried to "follow the price/cost", rather than "follow the moon".

Nil Einne (talk) 16:56, 8 October 2015 (UTC)

Yes, I checked those sources. However some of them are 7-8 years old. That's eternity for the Internet.

And by antipodes or moon, I mean it figuratively. You all are being too literal, even for the computing desk.

Maybe the whole thing is feasible for cryptocurrencies (like bitcoin) miners, but also in this case, they would be following the cents, as you point to. Maybe someone is just buying the night time of some servers for mining purposes.YX-1000A (talk) 17:37, 8 October 2015 (UTC)

It makes sense to crunch huge amounts of data, be it for mining bitcoins, for finding the next biggest prime, or for finding that message from space. However, keep in mind that this is not the same task that's being passed to other servers. Each server's admin get to choose how much resources he want to invest into the crunching, for whatever reasons. In principle you could have your server with 0% idle time if you wanted, independently of the kw price. However the admin will have to ask himself whether a task makes sense.Yppieyei (talk) 19:00, 8 October 2015 (UTC)

I think to answer the question, not common at all. The power it takes to run the servers is just a fraction of what it costs to run a data center, regardless of how much the servers are "working". Air conditioning, security, maintenance, etc, all have to be running before you even start installing servers.. If you take the above example, you have 2 data centers mining bitcoins, one is running during the night using cheap power, the other is sitting there idle during the day? But it will still cost you a considerable amount to maintain during the day, unless you literally shut everything down and turn the power off completely and tell everyone to leave, which for a high volume data center with redundant power and batteries, etc is not trivial. Vespine (talk)

For the vast majority of data centers, load is user-driven. There are a few that use thermal storage to lower the day-time energy use (using a reservoir of coolant, or producing ice at night). Ssscienccce (talk) 03:55, 9 October 2015 (UTC)

October 9

whatsapp on iphone

a friend told me that if you have an iphone and you receive a whatsapp message then somewhere in the system (lockscreen?) there shows up a preview of the whatsapp message, what I would like to know: is it possible to read the FULL message in that preview or does it just show like the first two sentences or so --87.189.238.167 (talk) 09:16, 9 October 2015 (UTC)