I tried
$\min_{x \in \mathbb{R}^m, \\ y \in \mathbb{R}^n}$
but \\ doesn't create a line-break in this setting
\\
\substack{}
\substack
:-)
\substack{}– develarist Dec 02 '20 at 08:54\substacktoo:-)– campa Dec 02 '20 at 08:56