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Latex piecewise function

How to write Latex piecewise function with left operator or cases environment.


First of all, modifiy your preamble adding

\usepackage{amsfonts}

Latex piecewise function with left operator

\begin{equation*}
y = f(x) = \lvert x \rvert = \left\{
       \begin{array}{ll}
           -x & \quad x \leq 0 \\
           x & \quad x \geq 0
       \end{array}
   \right.
\end{equation*}

$$ \begin{equation*} y = f(x) = \lvert x \rvert = \left\{ \begin{array}{ll} -x & \quad x \leq 0 \\ x & \quad x \geq 0 \end{array} \right. \end{equation*} $$

Latex piecewise function with cases environment

\begin{equation*}
g(x)=\begin{cases}
         x \quad &\text{if} \, x \in \mathbb{Q} \\
         -x \quad &\text{if} \, x \notin \mathbb{Q} \\
    \end{cases}
\end{equation*}

$$ \begin{equation*} g(x)=\begin{cases} x \quad &\text{if} \, x \in \mathbb{Q} \\ -x \quad &\text{if} \, x \notin \mathbb{Q} \\ \end{cases} \end{equation*} $$