f(x) = 
\begin{cases}
1 & \text{if } x \text{ is rational} \\ 
0 & \text{if } x \text{ is irrational}
\end{cases}

\lim\limits_{x \to c} f\left(x\right)

\begin{aligned}

\frac{x+y}{2} \geq \sqrt{xy} & \iff x+y \geq 2\sqrt{xy} \\
& \iff x^2 + 2xy + y^2 \geq 4xy \\
& \iff x^2 - 2xy + y^2 \geq 0 \\
& \iff \left(x-y\right)^2 \geq 0

\end{aligned}

\iff

\implies

\to \gets

\displaystyle\left.\frac{dy}{dx}\right\rvert_{{x=1 \atop y=2}}

\sqrt[p]{x}

x^{\frac{1}{p}}

\overset{ac}{\underset{ef}{bd}} g

{ac \atop bd} e

( a ), [ b ], \{ c \}, | d |, \| e \|, \langle f \rangle, \lfloor g \rfloor, \lceil h \rceil, \ulcorner i \urcorner

\hat{x} \bar{x} \tilde{x}

\displaystyle\sum\limits_{n=a }^{b} f(n)

\displaystyle\prod\limits_{n=a}^{b} f(n)

\displaystyle\int_a^b x; \iint_a^b x

a\cdots \ddots \vdots \ldots

\begin{bmatrix}
a_{1,1} & a_{1,2} & \cdots & a_{1,c} \\
a_{2,1} & a_{2,2} & \cdots & a_{2,c} \\
\vdots & \vdots & \ddots & \vdots \\
a_{r,1} & a_{r,2} & \cdots & a_{r,c}
\end{bmatrix}

\mathrm{Tex}\mathit{Ita}\mathbf{Bol}\mathsf{San}\mathtt{Mon}

\mathfrak{Fraktur}\mathcal{CALLIG}

\mathbb{BBOLD}\mathscr{SCRIPT}

a \bmod b

x \equiv a \pmod{b}

{\color{red}x} \mathbin{\color{blue}+} {\color{green}y}

{\color{red}\sqrt{\color{black}x^2+y^2}}+12

\int y\! \mathrm{d}x

\int y \mathrm{d}x

\int y\, \mathrm{d}x

\int y\: \mathrm{d}x

\int y\; \mathrm{d}x

\int y\quad \mathrm{d}x