https://en.wikibooks.org/wiki/LaTeX/Advanced_Mathematics#Advanced_formatting
高级数学公式编辑见上述链接
\begin{align*}
& \max v(f),\\
& s.t. \sum_{j:(v_i,v_j)\in A}f_{ij} - \sum_{j:(v_j,v_i)\in A}f_{ji} = \left\{
\begin{array}{ll}
v(f),i = s\\
-v(f),i = t\\
0,i\neq s,t\\
\end{array}
\right. \\
& 0 \leq f_{ij} \leq c_{ij},\forall (v_i,v_j) \in A
\end{align*}
去掉* 可以让公式带有编号
\begin{align}
& \max v(f),\\
& s.t. \sum_{j:(v_i,v_j)\in A}f_{ij} - \sum_{j:(v_j,v_i)\in A}f_{ji} = \lef