# Definition:Continuous Mapping (Topology)

*This page is about Continuous Mapping in the context of topology. For other uses, see Continuous Mapping.*

## Definition

Let $T_1 = \struct {S_1, \tau_1}$ and $T_2 = \struct {S_2, \tau_2}$ be topological spaces.

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

## Continuous at a Point

Let $x \in S_1$.

### Definition using Open Sets

The mapping $f$ is **continuous at (the point) $x$** (with respect to the topologies $\tau_1$ and $\tau_2$) if and only if:

- For every open set $U_2$ of $T_2$ such that $\map f x \in U_2$, there exists an open set $U_1$ of $T_1$ such that $x \in U_1$ and $f \sqbrk {U_1} \subseteq U_2$.

### Definition using Neighborhoods

The mapping $f$ is **continuous at (the point) $x$** (with respect to the topologies $\tau_1$ and $\tau_2$) if and only if:

- For every neighborhood $N$ of $\map f x$ in $T_2$, there exists a neighborhood $M$ of $x$ in $T_1$ such that $f \sqbrk M \subseteq N$.

### Definition using Neighborhood Inverse

The mapping $f$ is **continuous at (the point) $x$** (with respect to the topologies $\tau_1$ and $\tau_2$) if and only if:

- For every neighborhood $N$ of $\map f x$ in $T_2$, $f^{-1} \sqbrk N$ is a neighborhood of $x$.

### Definition using Filters

The mapping $f$ is **continuous at (the point) $x$** if and only if:

- for any filter $\FF$ on $T_1$ that converges to $x$, the corresponding image filter $f \sqbrk \FF$ converges to $\map f x$.

## Continuous on a Set

Let $S$ be a subset of $S_1$.

The mapping $f$ is **continuous on $S$** if and only if $f$ is continuous at every point $x \in S$.

## Continuous Everywhere

### 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 **$\tuple {\tau_1, \tau_2}$-continuous** can be used to define a **continuous mapping**.

## Also see

## Sources

- 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**continuous map**