Latex - cases function

 f(n) =
\begin{cases}
n/2,  & \text{if $n$ is even} \\
n+1, & \text{if $n$ is odd}
\end{cases}

\[f(n) = \begin{cases} n/2, & \text{if $n$ is even} \\ n+1, & \text{if $n$ is odd} \end{cases}\]