How to fix this error when running sudo apt-get update?
E: The repository 'http//archive.canonical.com precise Release' is not signed
I'd like to create a picture like the one with TikZ:    I've already created the electrical part using circuitikz. I'd like to add the part on the right (the motor's shaft with arrows), but don't know how to start, actually. Is there a way to draw 3D shapes, maybe? Or should I draw circles and rotate them?  \documentclass{article} \usepackage{tikz} \usetikzlibrary{intersections} \usepackage{circuitikz} \usepackage{siunitx}  \begin{document}  \begin{figure} \centering \begin{circuitikz} \draw (0,0) to [V, v=$v(t)$, *-*] (0,2)      to [R, l=$R_d$] (2,2)     to [L, l=$L_d$] (4,2)     to [V, v&lt;=$e(t)$] (4,0) to [generic] (4,2) -- (4,0) -- (0,0); \end{circuitikz} \end{figure}  \end{document}   What I have now:   What I'm still missing: Align Caption to the left MySQL replication of single DB to same server Changing order of features in shapefile I have a shapefile and I want to change the order of the features. Is that possible? I want to sort the features in the shapefile because I'm embedding this shapefile in a mobile app. The app will display a list of features, and although I could do the sorting at load time, I'd prefer to have them sorted. Where can I download Ubuntu 16.04? I am unable to find the link for downloading Ubuntu 16.04 onto my   desktop/laptop. I am able to see the link only for 18.04, but I need 16.04. Why am I getting black photos when I use the Manual feature on my DSLR? I was taking photos a few days ago, and I had it set to Manual. After about four photos, the rest started coming out black. I could see through my eyepiece clearly, and it took the photo normally, but when I reviewed it they were all black. Principal component analysis "backwards": how much variance of the data is explained by a given linear combination of the variables? I have carried out a principal components analysis of six variables $A$, $B$, $C$, $D$, $E$ and $F$. If I understand correctly, unrotated PC1 tells me what linear combination of these variables describes/explains the most variance in the data and PC2 tells me what linear combination of these variables describes the next most variance in the data and so on.   I'm just curious -- is there any way of doing this "backwards"? Let's say I choose some linear combination of these variables -- e.g. $A+2B+5C$, could I work out how much variance in the data this describes? Trouble doing polynomial interpolation Escaping quotes in zsh alias Following on from  about stripping newlines out of text, I want to turn this into a zsh alias as follows:  alias striplines=' awk " /^$/ {print \"\n\"; } /./ {printf( \" %s \",$0);}"'   I've tried escaping the quotes inside the awk script, but I'm getting this error:  awk: (FILENAME=bspsrobustness FNR=1) fatal: division by zero attempted   (The file is called bspsrobustness)  Is there a way to do what I want? I suppose I could turn this into an awk script rather than a zsh alias, is that my best option? How can I connect two different DC power sources together? How can I connecting two different DC power sources together? for example 15Vdc(5Amp) with 5Vdc(2Amp). I want to make one DC source (20Vdc,7Amp) by more powerful supplies. Add review queues to Android app I know there are some similar posts on adding review queues to the mobile web page. However, this is regarding the Stack Exchange Android app. I really would like to review posts and edits from the Android app, however this isn't included in the app yet. I'm not sure how it would be implemented; maybe there could be a button or something for review. Could we please see this? Why is it that many of the most important physical equations don't have ugly numbers (i.e., "arbitrary" irrational factors) to line up both sides?   Why can so many equations be expressed so neatly with small natural numbers while recycling a relatively small set of physical and mathematical constants?  For example, why is mass–energy equivalence describable by the equation $E = mc^2$ and not something like $E \approx 27.642 \times mc^2$?  Why is time dilation describable by something as neat as $t' = \sqrt{\frac{t}{1 - \frac{v^2}{c^2}}}$ and not something ugly like $t' \approx 672.097 \times 10^{-4} \times \sqrt{\frac{t}{1 - \frac{v^2}{c^2}}}$.  ... and so forth.    I'm not well educated on matters of physics and so I feel a bit sheepish asking this.   Likewise I'm not sure if this is a more philosophical question or one that permits a concrete answer ... or perhaps even the premise of the question itself is flawed ... so I would gratefully consider anything that sheds light on the nature of the question itself as an answer.    EDIT: I just wished to give a little more context as to where I was coming from with this question based on some of the responses:  @Jerry Schirmer comments:      You do have an ugly factor of $2.997458 \times 10^8 m/s$ in front of everything. You just hide the ugliness by calling this number c.   These are not the types of "ugly constants" I'm talking about in that this number is the speed of light. It is not just some constant needed to balance two side of an equation.  @Carl Witthoft answers:      It's all in how you define the units ...   Of course this is true, we could in theory hide all sorts of ugly constants by using different units on the right and the left. But as in the case of $E=mc^2$, I am talking about equations where the units on the left are consistent with the units on the right, irrespective of the units used. As I mentioned on a comment there:     $E=mc^2$  could be defined using units like $m$ in imperial stones ($\textsf{S}$), $c$ in cubits/fortnight ($\textsf{CF}^{−1}$) and $E$ in ... umm ... $\textsf{SC}^2\textsf{F}^{−2}$ ... so long as the units are in the same system, we still don't need a fudge factor.   When the units are consistent in this manner, there's no room for hiding fudge constants. I have an independent variable called "quality"; this variable has 3 modalities of response (bad quality; medium quality; high quality). I want to introduce this independent variable into my multiple linear regression. When I have a binary independent variable (dummy variable, I can code 0 / 1) it is easy to introduce it into a multiple linear regression model.  But with 3 modalities of response, I have tried to code this variable like this :  Bad quality      Medium quality      High quality       0                1                  0      1                0                  0      0                0                  1      0                1                  0   But there is a problem when I try to do my multiple linear regression: the modality Medium quality gives me NA:    Coefficients: (1 not defined because of singularities)    How can I code this variable "quality" with 3 modalities? Do I have to create a variable as a factor (factor in R) but then can I introduce this factor in a multiple linear regression? Why do we use the Lagrangian and Hamiltonian instead of other related functions? There are 4 main functions in mechanics. $L(q,\dot{q},t)$, $H(p,q,t)$, $K(\dot{p},\dot{q},t)$, $G(p,\dot{p},t)$. First two are Lagrangian and Hamiltonian. Second two are some kind of analogical to first two, but we don't use them. Every where they write that using them can cause problems. But why? The general advise is that you should not call GC.Collect from your code, but what are the exceptions to this rule?  I can only think of a few very specific cases where it may make sense to force a garbage collection.   One example that springs to mind is a service, that wakes up at intervals, performs some task, and then sleeps for a long time. In this case, it may be a good idea to force a collect to prevent the soon-to-be-idle process from holding on to more memory than needed.   Are there any other cases where it is acceptable to call GC.Collect? Require duplicates to be imported in Shotwell I have some drawings that I have imported and tagged by date. Now I want to take some of them that I have organized on my disk and put give them a new tag. Shotwell thinks they are duplicates and won't import them. Is there a way to require Shotwell to import these duplicates? How to check for an active Internet connection on iOS or macOS? Show $f$ is analytic if $f^8$ is analytic This is from Gamelin's book on Complex Analysis.  Problem: Show that if $f(z)$ is continuous on a domain $D$, and if $f(z)^8$ is analytic on $D$, then $f(z)$ is analytic on $D$. (I assume the intention is that $D$ is nice, i.e. open and connected)  I'm not exactly sure how to approach this. I'm guessing it has something to do with zeroes of $f$. For example, around non-zeroes, $f$ is analytic since $z^{1/8}$ is nonzero in a neighborhood. Or something along those lines. The details are eluding me.  Any help would be appreciated! Any evidence that subdomain can help with domain authority and vice-versa? How to execute .sh file on Windows? When I am trying to execute a file(name.sh) in the command line by the command  ./name.sh  , I am getting the error that:     "." is not recognized as an internal   or external command, operable or batch   file   please help me execute the .sh file