LaTeX

 \[ f(n) = \left\{ 
  \begin{array}{l l}
    n/2 & \quad \text{if $n$ is even}\\
    -(n+1)/2 & \quad \text{if $n$ is odd}
  \end{array} \right.\]

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