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Very new to Tex. Trying to get the most basic doc to work. Not matter what I do I always get an "Undefined control sequence" at line 1 of my document. As of now, I am only trying to build the following:  \documentclass{article} \begin{document} This is a test of the article \end{document}   Full error is:  This is pdfTeX, Version 3.1415926-2.5-1.40.14 (TeX Live 2013)  restricted \write18 enabled. entering extended mode (./example.tex ! Undefined control sequence. l.1 \documentclass                   {article} ?   I have tried many other variations, including \usepackage directives that people mentioned - nothing every varies. Prove that $2xy\mid x^2+y^2-x$ implies $x$ is a perfect square. Prove that $2xy\mid x^2+y^2-x$ implies $x$ is a perfect square. My work: $2xy\mid x^2+y^2-x \implies x^2+y^2-x=2xy\cdot k$ So,$x^2+y^2+2xy-x=(x+y)^2-x=2xy \cdot (k+1)$ And,$x^2+y^2-2xy-x=(x-y)^2-x=2xy \cdot (k-1)$ I found that for $x,y$ both odd, no solution exists. For $x$ even, and $y$ odd,no solution exists. Solution exists only for $x$ odd, $y$ even and $x$ even and $y$ even solution exists. Cannot do anything more. Please help! Solve the recurrence $T(n) = 2T(n-1) + n$ Uniformly Continuous Function sending Bounded Set to Unbounded One Let $(X,d)$ and $(Y,\rho)$ be metric spaces, and let $f: X \to Y$ be a uniformly continuous function. If $A \subset X$ is bounded, must $f(A) \subset Y$ be bounded?  It is clear to me that in metric spaces that satisfy the Heine-Borel property, such as $\mathbb{R}^{n}$, the answer to this question is yes. However, I can see no reason why this should hold for arbitrary metric spaces. Any ideas? Thanks! Sum of the series $\sum _{ r=1 }^{ \infty }{ \frac { { r }^{ 3 } }{ r! } } $ How do I prove that the sum of the series  $1+\frac { { 2 }^{ 3 } }{ 2! } +\frac { { 3 }^{ 3 } }{ 3! } +\frac { { 4 }^{ 3 } }{ 4! } +.... = 5e$ Let $R$ be a UFD and $r\in R$ irreducible. If $S=R-(r)$, why is $S^{-1}R$ a PID?   I've finished proving the integral domain part, which was pretty easy.  How to prove it's principal? consider $R \subset S^{-1}R \subset F$, where $F$ is the field of fractions of $R$?  thank you! :p If $p$ and $q = 2p + 1$ are both odd primes, show that $-4$ and $2(-1)^{(1/2)(p-1)}$ are both primitive roots modulo $q$. If $p$ and $q = 2p + 1$ are both odd primes, show that $-4$ and $2(-1)^{(1/2)(p-1)}$ are both primitive roots modulo $q$.   I cannot get heads nor tails of how to even start this let alone finish it Space after LaTeX commands Is a function defined at a single point continuous? Is a function defined at a single point continuous?  For example $f:\{0\}\to\{0\}$ defined by $f(x)=\sqrt{x}+\sqrt{-x}$ is a sum of two continuous functions and is therefore continuous, however for $f$ to be continuous, it has to have the limit $\lim\limits_{x_0\to x}f(x_0)=f(x)$ but it dosen't seem to have that limit. How do question bookmarks work? What does bookmarking a question mean? How do you bookmark a question? Do users get updates on their bookmarked questions? Are my bookmarks publicly visible? How do you see your own bookmarks? How do you see other users' bookmarks? How do you see which users have bookmarked a particular question? What impact does the bookmark system have on the voting system? Can I &quot;un-bookmark&quot; a question? Can I search within my bookmarks? Example of non-isomorphic structures which are elementarily equivalent I have a script in file bla.sh and it is executable. When I click on it, the script is executed and the window is closed. I'd like the window to stay open.  Something like command cmd /k** command in Windows.  P.S. I don't want to use pause, but I want to able to write more commands after the script was executed. Sound of a next scene starts one moment earlier than video What off-camera flash solution should I use with an entry-level Nikon? How to get the ASCII value of a character Which Harry Potter works are considered canon? There are many different Harry Potter works including:   books short stories movies card games video games toys amusement parks web content   Which ones are considered to be canon? What "allocation unit size" should I use for a drive with a single NTFS partition? I'm formatting a 1TB external hard drive as a single NTFS partition.  This drive is mainly meant for storing media.  Should I choose something other than the default allocation unit size?  The options range from 512 bytes to 64K.  Are there any guidelines that I should use with partitions storing other data?  Should I stop poking around and just leave it at "default"? Why is "bicycle" pronounced differently from other obviously related words? Thus is a picture of both of my hard-drives:    My C Drive and my D drive. I want to put Ubuntu onto my D Drive. Now my question here is if i do install onto my D Drive then how should I do it in order to retain use of the drive. I intend to partition about 60 gigs from it. Now after doing that I should still have about 830 gigs left. I want to still be able to use that in Windows. So how and will I be able to retain use of the drive after Installing? I have installed once before and I believe i did it wrong because not only could Windows not recognize my D drive but I also lost all the data on it.   My computer is a Lenovo Y700 It is a UEFI (I believe i got that right). How can I have a Linux machine join a Windows Domain? Sorry if I am misusing terms here; I actually don't know much about Active Directory and the related technologies.  Basically I have a Linux computer and I'd like it (or my user on that computer) to be associated with my user on the Domain, so that I can browse the network and all that stuff windows has.  Is this feasible?  What do I need to look into to do something like this?