LaTeX

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

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