# Definition:Homeomorphism/Metric Spaces

*This page is about topological equivalence of metric spaces. For other uses, see Definition:Topological Equivalence.*

## Contents

## Definition

### Definition 1

Let $M_1 = \left({A_1, d_1}\right)$ and $M_2 = \left({A_2, d_2}\right)$ be metric spaces.

Let $f: A_1 \to A_2$ be a bijection such that:

- $f$ is continuous from $M_1$ to $M_2$
- $f^{-1}$ is continuous from $M_2$ to $M_1$.

Then:

- $f$ is a
**homeomorphism** - $M_1$ and $M_2$ are
**homeomorphic**.

### Definition 2

Let $M_1 = \left({A_1, d_1}\right)$ and $M_2 = \left({A_2, d_2}\right)$ be metric spaces.

Let $f: A_1 \to A_2$ be a bijection such that:

- for all $U \subseteq A_1$, $U$ is an open set of $M_1$ if and only if $f \left[{U}\right]$ is an open set of $M_2$.

Then:

- $f$ is a
**homeomorphism** - $M_1$ and $M_2$ are
**homeomorphic**.

### Definition 3

Let $M_1 = \left({A_1, d_1}\right)$ and $M_2 = \left({A_2, d_2}\right)$ be metric spaces.

Let $f: A_1 \to A_2$ be a bijection such that:

- for all $V \subseteq A_1$, $V$ is a closed set of $M_1$ if and only if $f \left[{V}\right]$ is a closed set of $M_2$.

Then:

- $f$ is a
**homeomorphism** - $M_1$ and $M_2$ are
**homeomorphic**.

### Definition 4

Let $M_1 = \left({A_1, d_1}\right)$ and $M_2 = \left({A_2, d_2}\right)$ be metric spaces.

Let $f: A_1 \to A_2$ be a bijection such that:

- for all $a \in A_1$ and $N \subseteq A_1$, $N$ is a neighborhood of $a$ if and only if $f \left[{N}\right]$ is a neighborhood of $f \left({a}\right)$.

Then:

- $f$ is a
**homeomorphism** - $M_1$ and $M_2$ are
**homeomorphic**.

## Also known as

A **homeomorphism** is also known as a **topological equivalence**.

Two **homeomorphic** metric spaces can be described as **topologically equivalent**.

## Also see

- Results about
**homeomorphic metric spaces**can be found here.