I get the following output (and error at the end) when I run apt update on my system:  $ sudo apt update Ign:1 http://deb.debian.org/debian stretch InRelease                                                                                                                                                                                                          Hit:2 http://packages.microsoft.com/repos/vscode stable InRelease                                                                                                                  Ign:3 http://dl.google.com/linux/chrome/deb stable InRelease                                                                                                 Hit:4 http://security.debian.org/debian-security stable/updates InRelease Hit:6 http://ftp.us.debian.org/debian stable-updates InRelease                                                                                               Hit:7 http://fai-project.org/download stretch InRelease                                                               Hit:8 http://repository.spotify.com stable InRelease                                                                  Hit:9 http://deb.debian.org/debian-security stretch/updates InRelease                           Hit:10 http://dl.google.com/linux/chrome/deb stable Release               Hit:12 http://deb.debian.org/debian stretch-backports InRelease           Hit:13 https://download.docker.com/linux/debian stretch InRelease Hit:14 https://deb.nodesource.com/node_10.x stretch InRelease Ign:15 http://deb.debian.org/debian stable InRelease Ign:17 http://deb.debian.org/debian jessie-backports InRelease Ign:18 http://deb.debian.org/debian jessie InRelease Hit:19 http://deb.debian.org/debian testing InRelease Hit:5 https://release.memsql.com/production/debian memsql InRelease Hit:20 http://deb.debian.org/debian unstable InRelease Hit:21 http://deb.debian.org/debian stretch Release Hit:11 https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease Hit:23 http://deb.debian.org/debian stable Release   Err:25 http://deb.debian.org/debian jessie-backports Release   404  Not Found [IP: 151.101.248.204 80] Hit:26 http://deb.debian.org/debian jessie Release Reading package lists... Done E: The repository 'http://deb.debian.org/debian jessie-backports Release' does no longer have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details.   I don't understand why I'm getting this error.  $ cat /etc/apt/sources.list # These lines should work for many sites  deb http://deb.debian.org/debian stretch main contrib non-free deb http://deb.debian.org/debian-security stretch/updates main contrib non-free deb http://deb.debian.org/debian stretch-backports main contrib non-free  # repository that may contain newer fai packages for stretch deb [trusted=yes] http://fai-project.org/download stretch koeln  $ grep jessie-backports /etc/apt/sources.list.d/* /etc/apt/sources.list.d/jessie-backports.list:deb http://deb.debian.org/debian/ jessie-backports main contrib non-free /etc/apt/sources.list.d/jessie-backports.list:# deb-src http://deb.debian.org/debian/ jessie-backports main contrib non-free /etc/apt/sources.list.d/jessie-backports.list.save:deb http://deb.debian.org/debian/ jessie-backports main contrib non-free /etc/apt/sources.list.d/jessie-backports.list.save:# deb-src http://deb.debian.org/debian/ jessie-backports main contrib non-free   Some system information:  $ lsb_release -a No LSB modules are available. Distributor ID: Debian Description:    Debian GNU/Linux 9.8 (stretch) Release:        9.8 Codename:       stretch
I'm using a docker image as a base for my own development that adds the jessie backports repository in its Dockerfile and uses that to install a dependency. This image uses the following command to add the repository:  echo "deb http://ftp.debian.org/debian jessie-backports main" &gt;&gt; /etc/apt/sources.list   The problem is that fetching packages from the backports repository now  fails with the following error (this used to work previously):  W: Failed to fetch http://ftp.debian.org/debian/dists/jessie-backports/main/binary-amd64/Packages 404  Not Found  W: Failed to fetch http://deb.debian.org/debian/dists/jessie-updates/main/binary-amd64/Packages  404  Not Found   I looked on that server, and those paths are indeed not present there.  I tried to figure out on the Debian backports site whether this particular repository should still be available, and I didn't find any indication that this was deprecated or something like that.  Is this a temporary issue with the repository, or is the jessie-backports repository not available anymore? And if this is not a temporary issue, what options do I have to use this or an equivalent repository without upgrading to the newer Debian stable version?
ArrayList contains case sensitivity How do I use the latest GCC on Ubuntu? I want to compile my program with the latest version of gcc.  Ubuntu 14.04 comes with gcc 4.8.2, however there's 4.9.0 available, moreover, I see that it is available as a package: . I tried to install it  sudo apt-get install gcc-4.9   but it says  Reading package lists... Done Building dependency tree        Reading state information... Done Note, selecting 'gcc-4.9-base' for regex 'gcc-4.9' gcc-4.9-base is already the newest version. 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.   Looks like it is already installed, just not as the default one? How do I utilize it to build my program? I'm running a few VMs, and suddenly I noticed my CPU fan was spinning louder than usual. I tracked it down to Windows 8 doing some "automatic maintenance":      How can I turn this completely off? I found a way to set the time, but not a way to turn it off completely. Inexact measurement and wavefunction collapse As is usually said, measurement of an observable $q$ leads to collapse of wavefunction to an eigenstate of the corresponding operator $\hat q$. That is, now the wavefunction in $q$ representation is $\psi(q)=\delta(q-q_0)$ where $q_0$ is result of measurement.  Now, in reality measurements are never exact, so the wavefunction should not have that sharp peak. Instead it should be broadened depending on precision of measurement. In this case, can we still introduce an operator, an eigenstate of which would the new wavefunction appear? Is it useful in any way? Or does the new wavefunction depend too much on the way it was measured so that each instrument would have its own operator? How would such opeartor look e.g. for a single-slit experiment? Add a way to see where links go in Android app I think it would be good to have a way for the user to view the target URL of a link before following it. I'm pretty sure that I'm not the only one who dislikes blind-clicking -tapping links.   When I use SE in the mobile browser, I routinely long-press links to find out where they go before following them. I wish I could do the same thing in the app, with the same or a similar method.  (It's important that this won't conflict with text selection, since you can't long-press on links to select them now anyway.) Prove that for any positive integer $n$, $$2\sqrt{n + 1} − 2 \le 1 +\frac 1 {\sqrt 2}+\frac 1 {\sqrt 3}+ \dots +\frac 1 {\sqrt n} \le 2\sqrt n − 1$$ Progress I think Riemann sum should be used for the middle term. I got the limit as $n\to \infty$ of the function inside using Riemann sum but what I cannot get are the lower and upper bound. Environment variables in Mac OS X Update: The link below does not have a complete answer. Having to set the path or variable in two places (one for GUI and one for shell) is lame.  Not Duplicate of:     Coming from a Windows background where it's very easy to set and modify environment variables (just go to System Properties > Advanced > Environment Variables), it does not seem to be that straight forward on Mac OS 10.5. Most references say I should update /etc/profile or ~/.profile. Are those the equivalent of System Variables and User Variables? For example, where should I set my JAVA_HOME variable?    EDIT:  I want to be able to access the variable from the terminal as well as an app like Eclipse. Also, I hope I don't have to restart/logout to make this take effect. I wanted to apply texture on some house following an tutorial, I'm sure I did all what was needed but still don't see the texture.  Here's a screenshot: Place questions with X net downvotes on hold automatically To resolve the frequent issue of poor question unfairly gaining answers, and thus paving the way for more poor questions to be asked  I Propose  EDIT: From feedback, have changed net vote threshold to 5 (to at least match that of the current 5 users at 3k rep) END EDIT     If a question has 5 net downvotes, the question is put   on hold automatically, and no answers are allowed until the question   is resolved/has been edited and net votes are > -5.   The net vote threshold could be perhaps different, to avoid grey area questions being on hold unfairly, but this also needs to be high enough to allow for early action.  On hold note, something such as:     This question has been put on hold via community voting on the   question, and cannot receive answers. To resolve this, edit and   improve the question.   This proposal would assist in stopping poor questions from receiving answers, as currently questioners not wanting to put in any effort know they will likely get an answer eventually.  This could also help improve site quality by making people improve their questions before they can have answers, so answers are only given to decent questions. Problem in skew-symmetric matrix Let $A$ be a real skew-symmetric matrix. Prove that $I+A$ is non-singular, where $I$ is the identity matrix. In C#, what makes a field different from a property, and when should a field be used instead of a property? Existence of irreducible polynomials over finite field Let $F$ be a finite field. How do we prove that for each $n \in \mathbb{N}$ there is an irreducible polynomial of degree $n$?  One can assume that $F = \mathbb{F}_{p^m}$ where $p$ is prime. If $n \ge |F|$ then I can construct an irreducible polynomial, namely  $ p(x) = 1 + \prod_{j=1}^{|F|} ( x - a_j )$  where $a_j$ are all the field elements. It is clear that $p(x)$ has no roots in $F$.  This trick doesn't work for $n &lt; |F|$. A counter-example: Let $F = \mathbb{F}_3$ and $p(x) = 1 + (x-1)(x-2)$, then $p(1) = 1$, $p(2) = 1$, $p(0) = 1 + (-1)(-2) = 1 + 2 = 3 = 0 \pmod 3$.  I know there is a way to count them using the Möbius function $\mu(n)$ but I want a proof without it that just shows existence. Extract current viewing parameters from a 3D view? Find the value of $\sum_{k=1}^{n}k\binom{n}{k}$? `testl` eax against eax? I am trying to understand some assembly.  The assembly as follows, I am interested in the testl line:  000319df  8b4508        movl   0x08(%ebp), %eax   000319e2  8b4004        movl   0x04(%eax), %eax   000319e5  85c0          testl  %eax, %eax   000319e7  7407          je     0x000319f0     I am trying to understand that point of testl between %eax and %eax? I think the specifics of what this code isn't important, I am just trying to understand the test with itself - wouldn't the value always be true? How can I automatically download lyrics in iTunes? Is there a way to automatically download lyrics in iTunes? There is an alignment issue in the stackexchange.com search page .  The bottom bar is missing in the search input box.  Sample URL:   Screen shot for reference:    Firefox version: 54.0b8 (32-bit) OS Version: Windows 10 Pro  Also happens with Chrome latest version. Edit Mail rules manually The one of solution of $x^4 - 2y^2 = -1$ is $x = 1$ and $y = 1$. However, the solution $(1, 1)$  of $x^4 - 2y^2 = 1$ is failed. We know $x = 1$ and $y = 1$ is small integers and we can check by trail method. In case more solutions are existing or not how to check? What are the solutions of $x^4 - 2y^2 = 1$? No custom off topic message when flagging on SO I have less than 3000 rep on StackOverflow. Now that the new closing changes are live, I cannot supply a custom off topic message:    I can enter a custom off topic flag message without issue here on meta, though it only has three canned off topic reasons, rather than five.  I'm not sure if this is the same for users with enough rep to vote to close directly, but the lack of a custom message eliminates the ability to use the correct flag reason in some cases.  For example, I wanted to flag , which is simply off topic on SO. However, I can't use a custom message, and none of the canned ones are appropriate.    appears to be the most up to date official information on custom reasons, and indicates that they are remaining in play for now.  supports that.  If this is by design, how should I flag such questions? This is not a case worth bothering the mods over, so I don't want to use a custom diamond attention flag.