Synthetic Basis formed from Synthetic Sub-Basis

Theorem
Let $X$ be a set.

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

Define:
 * $\displaystyle \mathcal B = \left\{{\bigcap \mathcal A: \mathcal A \subseteq \mathcal S, \, \mathcal A \text{ is finite}}\right\}$

Then $\mathcal B$ 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:
 * $\displaystyle \bigcap \varnothing = X \in \mathcal B$

By Subset of Union: General Result, it follows that axiom B1 for a synthetic basis is satisfied.

We have that $\mathcal B \subseteq \mathcal P \left({X}\right)$.

Let $B_1, B_2 \in \mathcal B$. Then there exist finite $\mathcal A_1, \mathcal A_2 \subseteq \mathcal S$ such that:
 * $\displaystyle B_1 = \bigcap \mathcal A_1$
 * $\displaystyle B_2 = \bigcap \mathcal A_2$

It follows that:
 * $\displaystyle B_1 \cap B_2 = \bigcap \left({\mathcal A_1 \cup \mathcal A_2}\right)$

By Union Smallest, $\mathcal A_1 \cup \mathcal A_2 \subseteq \mathcal S$.

We have that $\mathcal A_1 \cup \mathcal A_2$ is finite.

Hence $B_1 \cap B_2 \in \mathcal B$, so it trivially follows that axiom B2 for a synthetic basis is satisfied.

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