[Tex/LaTex] Span formula over multiple pages.

amsmathequations

I've looked through the search but I couldn't find what I specifically want.

I have the following

$$\begin{split} Very long equation \end{split}$$


Now, I have half a page left where I want to place that equation, but it is a little bit longer. The result is that LaTeX places the formula on the next page. Is there a way so that LaTeX automatically splits the equation over the two pages?

Not if you use split. Section 3.9 of the amsmath package documentation says that
Certain equation environments wrap their contents in an unbreakable box, with the consequence that neither \displaybreak nor \allowdisplaybreaks will have any effect on them. These include split, aligned, gathered, and alignedat.
The same section explains how to get the effect you need, using a different environment such as align or multline.