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 . Also, let . is continuous at iff for every open subset of , is open in .

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.

### TheoremEdit

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