# Synthetic Basis formed from Synthetic Sub-Basis

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## Theorem

Let $X$ be a set.

Let $\SS$ be a synthetic sub-basis on $X$.

Define:

- $\ds \BB = \set {\bigcap \FF: \FF \subseteq \SS, \text{$\FF$ is finite} }$

Then $\BB$ is a synthetic basis on $X$.

## Proof

We consider $X$ as the universe.

Thus, in accordance with Intersection of Empty Set, we take the convention that:

- $\ds \bigcap \O = X \in \BB$

By Set is Subset of Union: General Result, it follows that:

- $\ds X \subseteq \bigcup \BB$

That is, axiom $(\text B 1)$ for a synthetic basis is satisfied.

We have that $\BB \subseteq \powerset X$.

Let $B_1, B_2 \in \BB$.

Then there exist finite $\FF_1, \FF_2 \subseteq \SS$ such that:

- $\ds B_1 = \bigcap \FF_1$
- $\ds B_2 = \bigcap \FF_2$

It follows that:

- $\ds B_1 \cap B_2 = \bigcap \paren {\FF_1 \cup \FF_2}$

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By Union is Smallest Superset, $\FF_1 \cup \FF_2 \subseteq \SS$.

We have that $\FF_1 \cup \FF_2$ is finite.

Hence $B_1 \cap B_2 \in \BB$, so it follows by definition that axiom $(\text B 2)$ for a synthetic basis is satisfied.

$\blacksquare$

## Note

Note that by this construction, *any* collection of subsets of $X$ can form a synthetic basis and thus generate a topology on $X$.

## Sources

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