I am currently using 18.04.   I wanted to install signal for desktop and I got this code from the site to install on Debian:  curl -s https://updates.signal.org/desktop/apt/keys.asc | sudo apt-key add - echo "deb [arch=amd64] https://updates.signal.org/desktop/apt xenial main" | sudo tee -a /etc/apt/sources.list.d/signal-xenial.list sudo apt update &amp;&amp; sudo apt install signal-desktop   Since doing that I can't use APT; whatever I do I get the same error:  E: The "sudo" type is unknown in line 2 of the source list /etc/apt/sources.list.d/signal-xenial.list E: The source list could not be read. Go to the configuration of the repositories to fix the problem. E: _cache-&gt; open () failed, please report.   I've tried to find solutions online but they want me to use the sources.list and I am not aware of how to navigate there.  My question has been closed as a duplicate but I can't get to grips with the sources.list; it's empty and I don't know how to manipulate it.  I cannot install or do anything because everything I want to do is stopped by the error:  Ignoring the file "some-ppa.list.save.1" in the directory "/etc/apt/sources.list.d/" because it has an incorrect file extension. Ignoring the file "some-ppa.list.save.1" in the directory " /etc/apt/sources.list.d/" because it has the incorrect extension.  The repository 'http://ppa.launchpad.net/baitsart/voice-commands/ubuntu bionic Release' does not have a Release file. Updating from such a repository can not be done securely, and is therefore disabled by default. See apt-secure (8) manpage for repository creation and user configuration details. The repository 'http://ppa.launchpad.net/grasch-simon- listens / simon / ubuntu bionic Release 'does not have a Release file.
Every once in a while I see users having issues to update due to errors like this:  E:Type 'ain' is not known on line 1 in source list /etc/apt/sources.list.d/some-ppa.list'   with varying types/line numbers/source list files (often after removing a PPA).  How can such an error be fixed?
Proving that if $G$ has a Hamiltonian circuit, then the line graph $L(G)$ also has a Hamiltonian Circuit I have a graph G = (V, E). I know that the line graph L(G) of G is defined as follows: each vertex in L(G) corresponds to an edge in G, and two vertices are connected by an edge in L(G) if and only if the corresponding edges in G are adjacent.  Now I am trying to (a) prove that if G has a Hamiltonian circuit, then L(G) also has a Hamiltonian circuit, and (b) give an example of a graph G such that G does not have a Hamiltonian circuit but L(G) does.  However, I'm extremely stuck on how to do this, and any help would be greatly appreciated. Thanks so much! I have a Godox X2Tc and a Godox X1Rc. I want to control my Canon Speedlite 430 EXII. (I also have a Godox V1, which is a beautiful thing!) I have the Canon Speedlite attached to the Godox receiver, so it should fire. It just wont! How do I set it up correctly so the receiver makes the Canon Speedlite fire?  Update: Michael C answered this question. I had the ID numbers set differently on the transmitter and receiver. I made them the same and now it all works. Perfect! Thanks Michael!! I have just asked a question on   I read from this meta answer  that suggested     From a desktop support perspective there are certainly cases where a   large(r) number of machines will be affected by something, in which   case it's appropriate for SF. There are then others that are isolated   to a single computer, these tend to be the ones I vote to move to SU.   Since my question is clearly not about a single computer and is external to a single computer, I turned to ServerFault. I thought the question is about how to replicate a network admin strategy on my own (home) network. My question is, however, marked as off-topic, and I did see that homenetwork questions are off-topic. But this is not a single computer issue either.  My question is:  Where should such home/small network questions be asked?  What's the real difference between "professional" networks and the smaller networks that I build for my home or lab with a handful computers?  -- UPDATE --  The question asked here is different from the said duplicate question  as that question is about the difference among computer science and programming SE sites. It has nothing to do with network administration, whether it's large or small networks. None of the two SE sites in question (Server Fault and Super User) is even mentioned in the question or in the 2 cited answers.  -- UPDATE2 --  This question is not a duplicate of , which was cited in the OP and was the cause of this question. That question does not cover the overlapping area of small networks, which could both be for work and for home network. Don't know what could it be but my daughter keeps on crying every night. She's sleeping and suddenly she starts crying. Hungry ? (She drinks her last bottle of milk around 8pm.) Maybe teething? (every night, don't think so) Miss being in mommy's and daddy's bed? (she cries even if she's in our bed) I'm out of ideas, and we are kind of exhausted! Any ideas what might be the reason for her to cry every night? bash fork: retry: Resource temporarily unavailable I am trying to run a shell script which will create process using a shell script. I get Resource temporarily unavailable error. how to identify which limit (memory/process/filecount) is creating this problem. Below is my ulimit -a results.  core file size          (blocks, -c) 0 data seg size           (kbytes, -d) unlimited scheduling priority             (-e) 0 file size               (blocks, -f) unlimited pending signals                 (-i) 563959 max locked memory       (kbytes, -l) unlimited max memory size         (kbytes, -m) unlimited open files                      (-n) 65535 pipe size            (512 bytes, -p) 8 POSIX message queues     (bytes, -q) 819200 real-time priority              (-r) 0 stack size              (kbytes, -s) unlimited cpu time               (seconds, -t) unlimited max user processes              (-u) 10000000 virtual memory          (kbytes, -v) unlimited file locks                      (-x) unlimited Close Lightning Modal by clicking anywhere outside of it Why are there two tensions on my free body diagram? Where has all the RAM gone? Is there a way to substitute aliases such that it is appended or prepended to an existing command in the shell?   For example, defining the alias:  alias execloc='/home/user/'   Prepending this to a command in the bash terminal:  cd exeloc/temp/somefolder   Can something like this be done, or is there a way to do it? Mclaurins with $e^{\sin(x)}$ To evaluate $e^{\sin(x)}$ I use the standard series $e^t$ and $\sin(t)$, combining them gives me:  $e^t = 1+t+\dfrac{t^2}{2!}+\dfrac{t^3}{3!}+\dfrac{t^4}{4!}+O(t^5)$  $\sin(t) = t-\dfrac{t^3}{3!}+O(t^5)$  $\therefore e^{\sin(x)} = 1+\sin(x)+\dfrac{\sin^2(x)}{2!}+\cdots+O(x^5)$  $\iff e^{\sin(x)}=1+\left(x-\dfrac{x^3}{6}\right)+\dfrac{\left(x-\frac{x^3}{6}\right)^2}{2}+\dfrac{x^3}{6}+O(x^5) = 1+x +\dfrac{x^2}{2}-\dfrac{x^4}{6} + O(x^5)$  In the last step, I only evaluate up to $\sin^3x$ term, everything above has a grade equal to or greater than $x^5$  However, I'm wrong. According to Wolfram, the series expansion for $e^{\sin x} = 1+x+\dfrac{x^2}{2}-\dfrac{x^4}{8}+O(x^5)$.  What did I do wrong? I can't seem to find what or where. Why does `ls -d *[!e]*` display all files instead of omitting all files that contain an e? ls -d *e* prints out all files containing an "e" somewhere in it. But the opposite, ls -d *[!e]* does not seem to work. Why exactly is that?  is it because ls takes * as its own argument and expands all the files?  I understand I can do: ls -d !(*e*) (in Bash with extglob set, or in ksh). How can I buy apps in the App Store without a credit card? Here there is my proof (quite short and easy) of a rather straightforward result. The text of this question comes from a previous question of mine, where I ended up working on a wrong inequality. Here things are fixed. Still, I would like to know:  if it is sound, because absolute value always creates me some problem, and if there is a shorter (neater) way to get the result (maybe without using contradiction).    Proposition: $|d(x,y)-d(y,z)| \leq d(x,z)$. Proof: We proceed by cases.  Case 1: $d(x,y)-d(y,z) \geq 0$. Assume by contradiction that $d(x,y)-d(y,z) &gt; d(x,z)$. Hence, $d(x,y)&gt; d(y,z) + d(x,z)$, contradicting the triangle inequality.  Case 2: $d(x,y)-d(y,z) &lt; 0$. Assume by contradiction that $ - d(x,y) + d(y,z) &gt; d(x,z)$. Hence, $d(x,y) &lt; d(y,z) - d(x,z)$. By applying the triangle inequality, we have that $d(y,x) \geq d(y,z) + d(x,z)$, which is by symmetry equivalent to  $d(x,y) \geq d(y,z) + d(x,z)$, obtaining the desired contradiction. $\square$     As always, any feedback is welcome. Thank you for your time. Why are RSA key sizes almost always a power of two? I know that other bit sizes are possible, e.g. this HTTPS server seems to have a 9000 bit key , but it's very rare that one sees a key not of size 1024, 2048, 4096, etc. bits in common usage - what is the reason for this? Is it just customary, or is there a cryptographic advantage? Can somebody help me understanding the fouries series coefficients?  I know that if we have:  $$f(t) = \sum_{n=1}^N A_n \sin(2\pi nt + Ph_n) \tag{where $Ph_n$ = phase}$$  And because of the $\sin(a+b)$ formula: $$ f(t) = \sin(2\pi nt + Ph) = \sin(2\pi nt )\cos(Ph_n) + \cos(2\pi nt )\sin(Ph_n)$$ Then:  $$\sum_{n=1}^N A_n \sin(2\pi nt + Ph) = \sum_{n=1}^N (A_n\sin(2\pi nt )\cos(Ph_n) + A_n\cos(2\pi nt )\sin(Ph_n))$$ And by definition: $$a_n = A_n\cos(Ph_n)$$ $$b_n = A_n\sin(Ph_n)$$ Then:  $$f(t) = \sum_{n=1}^N A_n \sin(2\pi nt + Ph) = \sum_{n=1}^N (a_n\sin(2\pi nt ) + b_n\cos(2\pi nt ))$$ Where does the $A_n$ in the first formula come from? Could somebody explain me where does the $\frac{a_0}{2}$ in the formula below come from? $$f(x) = \frac{a_0}{2}+\sum_{n=1}^\infty (a_n \cos(nx) + b_n \sin(nx))$$  And the most important:  How can I proof the formulas for the $a_n$ and $b_n$ coefficients?  PLEASE, I'm searchinf for these answers for DAYS. DNS Resolution for example.com - Can you explain? One whose solution never makes me happy is the following: $$y'(t)=\sin(y+t)\text{.}$$ I would start by substituting $z(t)=y(t)+t$ to get an ODE in $z(t)$, but then I'm not sure about how to substitute back my solution to check if it's correct or not$\dots$ How to get access to USB-storage from an application installed as Snap? Evaluating the sum $\Sigma \, \frac{1}{k(k+p)}$ fp - How to keep running sum in variable? I have this code in my LaTeX:   \FPset{totalHours}{0}  \newcommand{\entry}[5]{     \FPeval{totalHours}{(totalHours)+#5}     #1 &amp; #2 &amp; #3 &amp; #4 &amp; #5 \\ }   Where \entry is meant to be an entry in a time log. The 5th argument is a number representing the number of hours in that entry. I want to keep a running total of the hours in a variable I can print later on, totalHours.   This seems to do the \FPeval but then totalHours gets reset to 0 by the next time I write \entry. What is the correct way to keep a running total? Thanks.