1

I am using the cases environment multiple times within the align environment, inputting fairly long equations. This creates a lot of white space, which I would like to get rid of.

Any suggestions of how to page break within the cases environment, or a suitable alternative, would be most helpful. To be more specific, I know that inputting \allowdisplaybreaks in the preamble does not break the cases environment (as can be seen in the following MWE).

\documentclass[11pt,a4paper]{amsart}
\allowdisplaybreaks
\usepackage{enumerate,amssymb,amsmath}
\begin{document}

\begin{align*}
&\text{something}\\
&=
\begin{cases}
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
&\text{if A;}\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
&\text{if B.}\\
\end{cases}
\\
&=
\begin{cases}
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
&\text{if A;}\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
&\text{if B.}\\
\end{cases}
\\
&=
\begin{cases}
\displaystyle{+ 
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
&\text{if A;}\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
\\
\displaystyle{+
\sum_{i=1}^{\frac{1}{2}(k-6)}
\frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)}
\binom{\frac{n}{2}}{i}
\binom{\frac{n}{4}}{k-6i-1}v^{k-2i}}
&\text{if B.}\\
\end{cases}
\end{align*}

\end{document}
  • Please edit your question and post an example code which illustrates the problem. Without this it may be difficult for anyone to help you. – Ian Thompson Mar 17 '14 at 16:31
  • 2
    Actually, you could try using \allowdisplaybreaks (see section 3.9 of the amsmath package documentation). Failing that, we really need an example. Welcome to TeX.SX, by the way. – Ian Thompson Mar 17 '14 at 16:34
  • I agree with the answer/large comment by @IanThompson, this would benefit by a rethink. Formulate it in a different manner. As presented here it is very hard for the reader to understand. – daleif Mar 18 '14 at 11:01
1

In a situation like this, I would be inclined to rethink my notation rather than looking for a TeX based solution. Even if you find a means of creating cases-like environments that can break over pages, the result won't look good, and its readability will be poor. It's hard to make specific suggestions without seeing your actual equations, but if the term that you show occurs repeatedly, I would be inclined to define

r_{nk} = \frac{n^2-2n(k-3i+6)-4i}{n(2k+7i)},

because this would save a lot of space.

1

I am responding to the "suitable alternative" part of this question. I have had the same question, and the best answer I've been able to find so far is the answer to the following question: Tikz - How to overlay Decorations over longtable

Granted, this is far from ideal, but it is a possible alternative.

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