Real Analysis/Topological Continuity

Several properties of continuity on sets of real numbers can be extended by examining continuity from a Topological standpoint. In topology, an alternate definition (i.e. other than the standard "epsilon-delta" real analysis definition) is usually used. This definition applies to any function between sets, not just to metric spaces.

Definition Let $A\subseteq \mathbb {R}$. Also, let $f:A\to \mathbb {R}$. $f(x)$ is continuous at $x=c$ iff for every open subset $V$ of $f(A)$, $U\subseteq f^{-1}(V)$ is open in $A$.

It must be mentioned here that the term "Open Set" can be defined in much more general settings than the set of reals or even metric spaces; however, for use in Real Analysis, the definition of Open Set that you are already familiar with will definitely suffice.

Theorem

For any continuous function f:A->B, U compact => f(U) compact.