# Equivalence of Definitions of Topology

## Contents

## Theorem

The following definitions of the concept of **Topology** are equivalent:

### Definition 1

Let $S$ be a set.

A **topology on $S$** is a subset $\tau \subseteq \powerset S$ of the power set of $S$ that satisfies the open set axioms:

\((\text O 1)\) | $:$ | The union of an arbitrary subset of $\tau$ is an element of $\tau$. | ||||||

\((\text O 2)\) | $:$ | The intersection of any two elements of $\tau$ is an element of $\tau$. | ||||||

\((\text O 3)\) | $:$ | $S$ is an element of $\tau$. |

If $\tau$ is a **topology** on $S$, then $\struct {S, \tau}$ is called a topological space.

The elements of $\tau$ are called the open sets of $\struct {S, \tau}$.

### Definition 2

Let $S$ be a set.

A **topology on $S$** is a subset $\tau \subseteq \powerset S$ of the power set of $S$ that satisfies the following axioms:

\((\text O 1')\) | $:$ | The union of an arbitrary subset of $\tau$ is an element of $\tau$. | ||||||

\((\text O 2')\) | $:$ | The intersection of any finite subset of $\tau$ is an element of $\tau$. |

## Proof

### Definition 1 implies Definition 2

Let $\tau$ be a topology on $S$ by definition 1.

$\text O 1$ is the same as $\text O 1'$, so $\text O 1'$ holds for $\tau$.

From General Intersection Property of Topological Space:

- if $\text O 2$ holds, then $\text O 2'$ holds.

Thus $\tau$ is a topology on $S$ by definition 2.

$\Box$

### Definition 2 implies Definition 1

Let $\tau$ be a topology on $S$ by definition 2.

$\text O 1'$ is the same as $\text O 1$, so $\text O 1$ holds for $\tau$.

$\text O 2'$ states that the intersection of any finite subset of $\tau$ is an element of $\tau$.

This applies when the subset of $\tau$ contains exactly $2$ sets.

Thus $\text O 2$ is a direct consequence of $\text O 2'$.

Also as a consequence of $\text O 2$, it follows that the intersection of an empty subset of $\tau$ is an element of $\tau$.

From Intersection of Empty Set it follows that $S \in \tau$.

So $\text O 3$ is a direct consequence of $\text O 2'$.

Thus all the open set axioms hold.

Thus $\tau$ is a topology on $S$ by definition 1.

$\blacksquare$

## Sources

- 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.:
*Counterexamples in Topology*(2nd ed.) ... (previous): Notes: Part $\text I$: Basic Definitions: Section $1$. General Introduction: $1$