\begin{eqnarray[*]}
var_1 & rel_1 & eq1 \\
var_2 & rel_2 & eq2 \\
....
\end{eqnarray[*]}
The eqnarray environment is typically used to display a sequence of equations
or inequalities; it may also be used to manage spacing for long equations.
The \lefteqn command is useful in
this environment for splitting long equations over several lines.
It is very much like a three-column array environment, with position argument rcl, i.e., the columns are justified right, center, and left, respectively. (However, \multicolumn may not be used.)
Consecutive rows are separated by \\ commands and consecutive items within a row separated by an &. Any item may be empty, i.e., no text.
A separate equation number is placed on every line unless that line has a \nonumber command. The optional eqnarray* form does not generate any equation numbers.
A \label command anywhere within a row generates a reference to that row's number.