LaTeX:Symbols
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This article will provide a short list of commonly used LaTeX symbols.
Contents
Common Symbols
Operators
Finding Other Symbols
Here are some external resources for finding less commonly used symbols:

Detexify is an app which allows you to draw the symbol you'd like and shows you the code for it!

MathJax (what allows us to use on the web) maintains a list of supported commands.

The Comprehensive LaTeX Symbol List.
Operators
Relations
Symbol  Command  Symbol  Command  Symbol  Command 

\le  \ge  \neq  
\sim  \ll  \gg  
\doteq  \simeq  \subset  
\supset  \approx  \asymp  
\subseteq  \supseteq  \cong  
\smile  \sqsubset  \sqsupset  
\equiv  \frown  \sqsubseteq  
\sqsupseteq  \propto  \bowtie  
\in  \ni  \prec  
\succ  \vdash  \dashv  
\preceq  \succeq  \models  
\perp  \parallel  
\mid  \bumpeq 
Negations of many of these relations can be formed by just putting \not before the symbol, or by slipping an n between the \ and the word. Here are a few examples, plus a few other negations; it works for many of the others as well.
Symbol  Command  Symbol  Command  Symbol  Command 

\nmid  \nleq  \ngeq  
\nsim  \ncong  \nparallel  
\not<  \not>  \not=  
\not\le  \not\ge  \not\sim  
\not\approx  \not\cong  \not\equiv  
\not\parallel  \nless  \ngtr  
\lneq  \gneq  \lnsim  
\lneqq  \gneqq 
To use other relations not listed here, such as =, >, and <, in LaTeX, you may just use the symbols on your keyboard.
Greek Letters
Symbol  Command  Symbol  Command  Symbol  Command  Symbol  Command 

\alpha  \beta  \gamma  \delta  
\epsilon  \varepsilon  \zeta  \eta  
\theta  \vartheta  \iota  \kappa  
\lambda  \mu  \nu  \xi  
\pi  \varpi  \rho  \varrho  
\sigma  \varsigma  \tau  \upsilon  
\phi  \varphi  \chi  \psi  
\omega 
Symbol  Command  Symbol  Command  Symbol  Command  Symbol  Command 

\Gamma  \Delta  \Theta  \Lambda  
\Xi  \Pi  \Sigma  \Upsilon  
\Phi  \Psi  \Omega 
Arrows
Symbol  Command  Symbol  Command 

\gets  \to  
\leftarrow  \Leftarrow  
\rightarrow  \Rightarrow  
\leftrightarrow  \Leftrightarrow  
\mapsto  \hookleftarrow  
\leftharpoonup  \leftharpoondown  
\rightleftharpoons  \longleftarrow  
\Longleftarrow  \longrightarrow  
\Longrightarrow  \longleftrightarrow  
\Longleftrightarrow  \longmapsto  
\hookrightarrow  \rightharpoonup  
\rightharpoondown  \leadsto  
\uparrow  \Uparrow  
\downarrow  \Downarrow  
\updownarrow  \Updownarrow  
\nearrow  \searrow  
\swarrow  \nwarrow 
(For those of you who hate typing long strings of letters, \iff and \implies can be used in place of \Longleftrightarrow and \Longrightarrow respectively.)
Dots
Symbol  Command  Symbol  Command  

\cdot  \vdots  
\dots  \ddots  
\cdots  \iddots 
Accents
Symbol  Command  Symbol  Command  Symbol  Command 

\hat{x}  \check{x}  \dot{x}  
\breve{x}  \acute{x}  \ddot{x}  
\grave{x}  \tilde{x}  \mathring{x}  
\bar{x}  \vec{x} 
When applying accents to i and j, you can use \imath and \jmath to keep the dots from interfering with the accents:
Symbol  Command  Symbol  Command 

\vec{\jmath}  \tilde{\imath} 
\tilde and \hat have wide versions that allow you to accent an expression:
Symbol  Command  Symbol  Command 

\widehat{7+x}  \widetilde{abc} 
Others
Command Symbols
Some symbols are used in commands so they need to be treated in a special way.
Symbol  Command  Symbol  Command  Symbol  Command  Symbol  Command 

\textdollar or $  \&  \%  \#  
\_  \{  \}  \backslash 
(Warning: Using $ for will result in . This is a bug as far as we know. Depending on the version of this is not always a problem.)
European Language Symbols
Symbol  Command  Symbol  Command  Symbol  Command  Symbol  Command 

{\oe}  {\ae}  {\o}  
{\OE}  {\AE}  {\AA}  {\O}  
{\l}  {\ss}  !`  
{\L}  {\SS} 
Bracketing Symbols
In mathematics, sometimes we need to enclose expressions in brackets or braces or parentheses. Some of these work just as you'd imagine in LaTeX; type ( and ) for parentheses, [ and ] for brackets, and  and  for absolute value. However, other symbols have special commands:
Symbol  Command  Symbol  Command  Symbol  Command 

\{  \}  \  
\backslash  \lfloor  \rfloor  
\lceil  \rceil  \langle  
\rangle 
You might notice that if you use any of these to typeset an expression that is vertically large, like
 (\frac{a}{x} )^2
the parentheses don't come out the right size:
If we put \left and \right before the relevant parentheses, we get a prettier expression:
 \left(\frac{a}{x} \right)^2
gives
And with system of equations:
\left\{\begin{array}{l}x+y=3\\2x+y=5\end{array}\right.
Gives
See that there's a dot after \right. You must put that dot or the code won't work.
And, if you type this
\underbrace{a_0+a_1+a_2+\cdots+a_n}_{x}
Gives
Or
\overbrace{a_0+a_1+a_2+\cdots+a_n}^{x}
Gives
\left and \right can also be used to resize the following symbols:
Symbol  Command  Symbol  Command  Symbol  Command 

\uparrow  \downarrow  \updownarrow  
\Uparrow  \Downarrow  \Updownarrow 
MultiSize Symbols
Some symbols render differently in inline math mode and in display mode. Display mode occurs when you use \[...\] or $$...$$, or environments like \begin{equation}...\end{equation}, \begin{align}...\end{align}. Read more in the commands section of the guide about how symbols which take arguments above and below the symbols, such as a summation symbol, behave in the two modes.
In each of the following, the two images show the symbol in display mode, then in inline mode.
Symbol  Command  Symbol  Command  Symbol  Command 

\sum  \int  \oint  
\prod  \coprod  \bigcap  
\bigcup  \bigsqcup  \bigvee  
\bigwedge  \bigodot  \bigotimes  
\bigoplus  \biguplus 