Vertical spacing and page breaks in multiline displays

You can use the
|[|\  between lines, unless displaybreak or allowdisplaybreaks is used. The philosophy is that page breaks in such situations should receive individual attention from the author. displaybreak is best placed immediately before the
where it is to take effect. Like /'s pagebreak, displaybreak takes an optional argument between 0 and 4 denoting the desirability of the pagebreak. |[0]| means ``it is permissible to break here'' without encouraging a break; displaybreak with no optional argument is the same as |[4]| and forces a break.

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 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. When display breaks are enabled with allowdisplaybreaks, the
* command can be used to prohibit a pagebreak after a given line, as usual.