inside a align environment I have a limes with two variables which should be displayed in two lines:

\begin{align} ... f & = \lim _ {a \rightarrow -\infty \linebreak b \rightarrow \infty} ... \ ... \end{align} 

Currently there is no linebreak where i want to have one, it's displayed in one line, how can i force a line break there? \linebreak is ignored and \\ breaks the align environment.

Thanks for your help!

Florian