\lefteqn

 

  \lefteqn{eqn}

 

prints eqn in display math style, but pretends that it has zero width. It is typicaly used within an Eqnarray environment for displaying long equations that require multiple lines.

 

A typical use might be

  \begin{eqnarray}
  \lefteqn{\int_0^\infty some expression =} \\
  & & some terms \\
  & & \mbox{} + more terms
  \end{eqnarray}

Note that the empty \mbox{} is so LaTeX knows that the following + is a binary operator, and is not attached to the first of the "more terms" as a unary operator.

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