problem of introducing long brackets in different lines

Question

I have a problem writing an equation in eqnarray environment, where I split a single line into two lines. The first line starting with \left[ and the second line ending with \right], but I could not compile it using latex.

Answer 1

You should use align instead ofeqnarray as per this question that compares eqnarray vs align, and Stefan Kottwitz blong entry: \eqnarray vs \align environment.

Here is an example of using the \right., and \left. pair to complete the matching pair using the align environment. Note that the size of the brackets is not the same in the first example. This is due to the fact that the \left. <math> \right] of the second line does not see the vertical spacing of the \left[ <math> \right. that the first line does. To fix that you need to add a \vphantom{} with the term which has the largest vertical spacing in the first line. This yields the second result:

\documentclass{standalone}
\usepackage{amsmath}
\begin{document}
\begin{align*}
  y &= \left[\frac{1}{2}\right. \\
    &\qquad + \left.x^2+c\right]
\end{align*}
%
\begin{align*}
  y &= \left[\frac{1}{2}\right. \\
    &\qquad + \left.x^2+c\vphantom{\frac{1}{2}}\right]
\end{align*}
\end{document}

Answer 2

Another option would be to use the big-g delimiters instead of the \left, \right construct; using the big-g delimiters you don't have to worry for pairing the symbols in every line:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align*}
  y &= \biggl[ \frac{1}{2} \\
    &+ x^2 + c \biggr]
\end{align*}

\end{document}

Answer 3

you can use the breqn package.

instead of \begin{equation} or \begin{equation*}, use the environment dmath or dmath*. the delimiters will be sized properly, and the line broken in an appropriate location. there are also multi-line environments if your display needs them.

there are some limitations; for details, see the package documentation -- texdoc breqn if you have a tex live installation; otherwise, look on ctan.

Answer 4

Another option is to use nath class. It is not completely compatible with amsmath, but does provide its own multi-line math environments, which might be sufficient for simple math displays.

One of the features of nath is automatic delimiter scaling that works across multiple lines. For example

\documentclass{article}
\usepackage{nath}

\begin{document}
\begin{equation}
  y = \wall [ \sum_{i=1}^n a_i \\
            + x^2 + c ] \return
\end{equation}
\end{document}

gives