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Why is the ampere a base unit and not the coulomb? I always thought of current as the time derivative of charge, $\frac{dq}{dt}$. However, I found out recently that it is the ampere that is the base unit and not the coulomb. Why is this? It seems to me that charge can exist without current, but current cannot exist without charge. So the logical choice for a base unit would be the coulomb. Right? We should make books of our best content! And that's a good idea!  We have a lot of high quality content here, certainly enough for a small book.   I'm not saying that it necessarily has to be integrated into the main site, but this is just a suggestion if anyone wants to implement it.   Stack Exchange probably could make money selling the physical books though. CC-BY-SA allows selling, as long as you allow others to make copies of your book, and credit the author.  What Wikipedia does is you can custom order any set of articles as a book (it even has a nice set of authors at the end, to satisfy the BY part of CC-BY-SA).  This could even tie into the docs feature. Using `M` instead of `k` when rep is >= 1M on https://chat.stackexchange.com/users I read on  ()    Shouldn't it be 1.03m or (if we really follow the conventions of the metric system) 1.03M instead? Given the Lie algebra, what is the systematic way to construct the matrix representation of the generators of the desired dimension? I ask this question here because it is the physicists for whom representation of groups is more important than mathematicians.   Let us, for example, take $SU(2)$ for concreteness. Starting from the generic parametrization of a $3\times 3$ unitary matrix $U$ with $\det U=1$, and using the formula of generators $$J^i=-i\Big(\frac{\partial U}{\partial \theta_i}\Big)_{\{\theta_i=0\}}$$ one can find the $3\times 3$ matrix representation of the generators.  However, I'm looking for something else.    Given the Lie algebra $[J^i, J^j]=i\epsilon^{ijk}J^k$, is there a way that one can explicitly construct (not by guess or trial) the $3\times 3$ representations of $\{J^i\}$? Will the same procedure apply to solve other Lie algebras appearing in physics such as that of $SO(3,1)$ (or $SL(2,\mathbb{C})$)? PGFplots foreach equivalent to TikZ's with multiple variables separated by a slash How to remove Ubuntu from dual boot system with Windows? I wish to remove Ubuntu and I'm not quite sure how. I know it has something to do with partitions. I have 3 partitions. 1. OS [683 GB]  2. Recovery [14 GB] 3. Ubuntu 12.04 LTS (E:)  I know it says Ubuntu but when I installed Ubuntu I think I put 14 gigabytes for it so I think its the recovery one but I'm not sure. I could have sworn I put more than 700 megabytes for it.  I have googled this and I can't seem to find an answer. Any help would be nice. How to Fix Broken Packages? How do I fix broken packages when it is not working? When I try to fix broken packages, it tells me to "fix broken packages." I've just started to play with pgfplot. I'm trying to prepare a plot like to following   \begin{figure}[h] \centering \begin{tikzpicture} \begin{axis}[minor tick num=1,  axis x line=middle, axis y line=middle, xlabel=$k$, ylabel=$\omega$, ymin=0,xmin=-2,ymax=60] \addplot[blue,domain=0:5, line width=1.5pt] {x^2+25}; \addplot[orange,domain=0:5,line width=1pt]{3*x+22.75}; \addplot[cyan,domain=0:1.5, line width=1pt,dashed]{18.166*x}; \node[coordinate,pin=right:{$v_{\phi}=\frac{\omega}{k}$}]     at (axis cs:1,17) {}; \node[coordinate,pin=right:{$v_{g}=\frac{d\omega}{dk}$}]     at (axis cs:2.5,30) {}; \node[coordinate,pin=left:{dispersion relation}]     at (axis cs:4.5,47) {}; \end{axis} \end{tikzpicture} \caption{Wave dispersion relation with phase and group velocity} \label{fig:2} \end{figure}   What I would like to do is dropping the x-y tick labels so that we just maintaing the x-y title. How can I obtain it? Is it correct to omit preposition before date? is uniform convergent sequence leads to bounded function? Suppose there is there is uniform convergent sequence $(f_n)$ on the set $A$, and each $f_n$ is bounded on $A$, i.e., there exist $M_n&gt;0$ such that $|f_n(x)|\le M_n$ for all $x\in A$ Is it true that there exist a function $f$ that is bounded on set A? Can I prove it this way:  Since $f_n \to f$ uniformly on A, $\exists n \ge N$ such that $$|f_N(x) - f(x)| \le 1$$ and $$|f_n(x)-f_N(x)| \le 1$$  Therefore, $|f(x)| \le |f_N(x)|+1 \le M_N + 1$ and $|f_n(x)| \le |f_N(x)| + 1 \le M_N + 1$, $x \in A , n \ge N$  Let $M = \max\{M_1, M_2,\dots,M_N\} + 1$ : $|f_n(x)| \le M$, $\forall x\in A$  Therefore, $f$ is a bounded function on A. multi-layer board copper pour I have a 6-layer board where the internal 4 planes are +15,GND,VCC,-15.  I was wondering if there was any advantage to doing a copper pour on the top and bottom layers?  I would probably leave them floating as I don't want to use micro-vias to say tie it to GND?    Is this actually a bad idea?  I.e. floating copper=antenna.  Would it be just as acceptable to have 4 layer board with the top layer having copper pour to VCC and the bottom having a pour to GND and keep the two internals as +-15?  Note that this is for a fairly low speed circuit which has some analog and digital parts. Autofilling fields based on another field How to prove that $A_5$ has no subgroup of order 30? Stacking symbols Help with a proof. Countable sets. Where to find Birth and Death Records for Bucharest, Romania? My husband's family came from Bucharest. Specifically his mother was born in Sibiu.  I have been able to piece together names and relations of several grand and great grandparents from word of mouth and some fortunate pictures they had of the cemetery in Sibiu.  I cannot however find any records of birth's and death's for any of them.   Are there sites or organizations that hold records for Romania? I'm trying to include a link to some code on OneDrive; the link includes an underscore, which is causing a lot of trouble.  I tried adding the entry like this:  @misc{code,     title = {Code},     url = {https://1drv.ms/link_link} }   I use \usepackage{url} and \bibliographystyle{IEEEtran}. Everything compiles fine and the bibliography shows the correct entry. However, if you click the link, it opens a page at https://1drv.ms/link - so ignoring the underscore and everything after it, which leads to a wrong page. Even copy and paste from the PDF doesn't work since the underscore is copied as space, which again is a broken link.  How can I add the link such that it can be clicked and copied properly? I tried \_, \usepackage{underscore}, {{link}}, different bibtex entries but nothing worked. Also all the related questions/answers don't answer mine. I am interested in theoretical and practical considerations. Let's say we are given a set of positive reals, and we're told that these are the edges of a convex  $n$-gon, and we must compute it's area.   For $n = 3$ there is the famous Heron's formula:  $$A = {1\over 4}\sqrt{4a^2b^2 - (a^2 + b^2 - c^2)^2}$$  For $n = 4$ there is Brahmagupta's formula:  $$A = {1\over 4}\sqrt{(a^2 + b^2 + c^2 + d^2)^2 + 8abcd - 2(a^4 + b^4 + c^4 + d^4)}$$  Is there a generalization for arbitrary $n$? Does it even exist? Convert a column into a comma separated list