# Definition:Lower Semicontinuous/Subset

Let $f: S \to \R \cup \left\{{-\infty, \infty}\right\}$ be an extended real valued function.
Let $S$ be endowed with a topology $\tau$.
Let $A \subseteq S$, and $A \ne \varnothing$.
The function $f$ is said to be lower semicontinuous on $A$ if and only if $f$ is lower semicontinuous at every $a \in A$.