a \not = b
$$ a \not = b$$
\sum_{i=i}^{n}x_i = x_i + x_2 + ... + x_n
$$ \sum_{i=i}^{n}x_i = x_i + x_2 + ... + x_n $$
x_{ij} = \left\{
\begin{array}{ll}
1 & \mbox{si $j \not = i$,} \\
0 & \mbox{si $j=i$}
\end{array}
\right.
$$ x_{ij} = \left\{ \begin{array}{ll} 1 & \mbox{si $j \not = i$,} \\ 0 & \mbox{si $j=i$} \end{array} \right. $$
[201],[202].