Possible Duplicate:
How can I split an equation over two lines
I have a long equation,
$\operatorname{E} (Z_{n+1} - Z_n | X_1,..., X_n)
= \operatorname{E} (S_{n+1}^2 - (n+1) \sigma^2 - S_n^2 + n \sigma^2 | X_1,..., X_n)
= \operatorname{E} (S_{n+1}^2 - S_n^2 - (n+1) \sigma^2 + n \sigma^2 | X_1,..., X_n)
= \operatorname{E} (X_{n+1}(X_{n+1} + 2\sum_{i=1}^n X_i) - \sigma^2 | X_1,..., X_n)
= \operatorname{E} (X_{n+1}X_{n+1}) + 2\operatorname{E} (X_{n+1}) \sum_{i=1}^n X_i - \sigma^2
= \sigma^2 - \sigma^2 =0
$.
which I think causes warning:
Overfull \hbox (3.16257pt too wide)
Is there some way automatically line-break long equations? Or must I specify where to line-break? Thanks!