Letex中用大括号写公式不能换行

用cases不能换行，可以采用如下：

\begin{equation}\label{equ1}
P_r(l_i) =
\left\{
 \begin{array}{ccc}G_\sigma(1-(\lambda_1-\lambda_2))G_\sigma(\lambda_2-\lambda_3)G_\sigma(\lambda_3), l_i = 1 \\
G_\sigma(1-(\lambda_1-\lambda_2))G_\sigma(\lambda_2-\lambda_3)G_\sigma(\lambda_3),l_i = 2 \\
G_\sigma(1-(\lambda_1-\lambda_2))G_\sigma(\lambda_2-\lambda_3)G_\sigma(\lambda_3),l_i = 3
 \end{array}
  \right.
\end{equation}