# Continuous Bijection from Compact to Hausdorff is Homeomorphism

## Contents

## Theorem

Let $T_1$ be a compact space.

Let $T_2$ be a Hausdorff space.

Let $f: T_1 \to T_2$ be a continuous bijection.

Then $f$ is a homeomorphism.

### Corollary

Let $T_1$ be a compact space.

Let $T_2$ be a Hausdorff space.

Let $f: T_1 \to T_2$ be a continuous injection.

Then $f$ determines a homeomorphism from $T_1$ to $f \sqbrk {T_1}$.

That is, $f$ is an embedding of $T_1$ into $T_2$.

## Proof

Let $g = f^{-1}$.

We need to show that $g: T_2 \to T_1$ is continuous.

For any $V \subseteq T_1$, we have $g^{-1} \left({V}\right) = f \left({V}\right)$.

We are to show that if $V$ is closed in $T_1$, then $g^{-1} \left({V}\right)$ is closed in $T_2$.

Suppose $V$ is closed in $T_1$.

Since $T_1$ is compact, $V$ is compact by Closed Subspace of Compact Space is Compact.

So $f \left({V}\right)$ is compact from Continuous Image of Compact Space is Compact.

Since $T_2$ is Hausdorff, $f \left({V}\right)$ closed by Compact Subspace of Hausdorff Space is Closed.

But $f \left({V}\right) = g^{-1} \left({V}\right)$, so $g^{-1} \left({V}\right)$ is closed.

From Continuity Defined from Closed Sets, it follows that $g$ is continuous.

Thus by definition, $f$ is a homeomorphism.

$\blacksquare$

## Sources

- 1953: Walter Rudin:
*Principles of Mathematical Analysis*... (previous) ... (next): $4.17$ - 1975: W.A. Sutherland:
*Introduction to Metric and Topological Spaces*... (previous) ... (next): $5.9$: An inverse function theorem: Theorem $5.9.1$