Reference. Recession cone and max-min of a function.

Question

I am looking for some bibliographic references where I can find a relationship between recession cone of a set $U\subset\mathbb{R}^n$ and the minimum / maximum of a function $f\colon U\to\mathbb{R}^n$. For example "if $a\in U$ is min of $f$ then $U_\infty$ is ..."