eqnarray
--------
\begin{eqnarray}
math formula 1 \\
math formula 2 \\
.
.
.
\end{eqnarray}
The `eqnarray' environment is used to display a sequence of
equations or inequalities. It is very much like a three-column `array'
environment, with consecutive rows separated by `\\' and consecutive
items within a row separated by an `&'.
An equation number is placed on every line unless that line has a
`\nonumber' command.
The command `\lefteqn' is used for splitting long formulas across
lines. It typesets its argument in display style flush left in a box of
zero width.