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I have a one-line statement of a theorem and a page-long proof of it. I have kept the proof inside

\begin{align*}
...
\end{align*}

Since there is no break TeXMaker keeps the whole proof in a single page. As a result, the previous page looks like: Theorem 2.2 in the first line, and the statement in the last line. The middle of the page shows a great white eternal void and the output shows the warning message:

"Underfull \vbox has occured while \output is active []"

How do I get rid of this? Is there anyway for the align environment to understand the turn of a page and generate auto-pagebreak? Is there any alternative to align which does not have this difficulty?

  • Welcome to TeX.SE! – Cragfelt Dec 27 '17 at 4:38
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    Is there a reason you can't use a \proof environment? Then you could use several align* environments that will break naturally. I think it's likely that a full-page calculation using align* could be improved in clarity by putting some explanatory prose in the middle. – Sandy G Dec 27 '17 at 5:26
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See the section “Vertical spacing and page breaks in multiline displays” in the amsmath documentation (texdoc amsmath or texdoc amsldoc). It says:

When the amsmath package is in use page breaks between equation lines are normally disallowed; the philosophy is that page breaks in such material should receive individual attention from the author. […]

If you prefer a strategy of letting page breaks fall where they may, even in the middle of a multi-line equation, then you might put \allowdisplaybreaks[1] in the preamble of your document. An optional argument 1–4 can be used for finer control: [1] means allow page breaks, but avoid them as much as possible; values of 2,3,4 mean increasing permissiveness.

You can read the documentation for more options.

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It remains to write \allowdisplaybreaks.

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