# Definition:Continuous Mapping (Topology)/Everywhere

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## Definition

Let $T_1 = \left({S_1, \tau_1}\right)$ and $T_2 = \left({S_2, \tau_2}\right)$ be topological spaces.

Let $f: S_1 \to S_2$ be a mapping from $S_1$ to $S_2$.

### Definition by Pointwise Continuity

The mapping $f$ is **continuous everywhere** (or simply **continuous**) if and only if $f$ is continuous at every point $x \in S_1$.

### Definition by Open Sets

The mapping $f$ is **continuous on $S_1$** if and only if:

- $U \in \tau_2 \implies f^{-1} \sqbrk U \in \tau_1$

where $f^{-1} \sqbrk U$ denotes the preimage of $U$ under $f$.

## Also known as

If it is necessary to distinguish between multiple topologies on the same set, then the terminology **$\left({\tau_1, \tau_2}\right)$-continuous** can be used for the above.

## Also see

- Results about
**continuous mappings**can be found here.