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eqnarray

The eqnarray environment is obsolete. It has infelicities, including spacing that is inconsistent with other mathematics elements. (See “Avoid eqnarray!” by Lars Madsen https://tug.org/TUGboat/tb33-1/tb103madsen.pdf). New documents should include the amsmath package and use the displayed mathematics environments provided there, such as the align environment. We include a description only for completeness and for working with old documents.

Synopsis:

\begin{eqnarray} 
  first formula left  &first formula middle  &first formula right \\
  ...
\end{eqnarray}

or

\begin{eqnarray*} 
  first formula left  &first formula middle  &first formula right \\
  ...
\end{eqnarray*}

Display a sequence of equations or inequalities. The left and right sides are typeset in display mode, while the middle is typeset in text mode.

It is similar to a three-column array environment, with items within a row separated by an ampersand (&), and with rows separated by double backslash \\). The starred form of line break (\\*) can also be used to separate equations, and will disallow a page break there (see \\).

The unstarred form eqnarray places an equation number on every line (using the equation counter), unless that line contains a \nonumber command. The starred form eqnarray* omits equation numbering, while otherwise being the same.

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.

This example shows three lines. The first two lines make an inequality, while the third line has not entry on the left side.

\begin{eqnarray*}
  \lefteqn{x_1+x_2+\cdots+x_n}     \\
    &\leq &y_1+y_2+\cdots+y_n      \\
    &=    &z+y_3+\cdots+y_n
\end{eqnarray*}