To get a bunch of equations all lined up nicely, you can do it like this
with \eqnarray*
:
\begin{eqnarray*} i_w\cdot\psi_v\cdot N_w&=&\psi_w\cdot i_w\cdot N_w\\ &=&\psi_w\cdot i_w\cdot\sum_{\sigma\in G_w}\sigma\\ &=&\psi_w\cdot\sum_{\sigma\in G_w}\sigma\\ &=& i_w\cdot N_w\cdot\psi_w. \end{eqnarray*}yields
A few things to notice are:
&
surrounds the symbol at which the equations are lined up--in
this case, the = sign.\\
, which denotes a carriage
return.
For more possibilities in formatting your equations, consult the Companion.