Definition:Basis (Topology)/Synthetic Basis/Definition 1

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Let $S$ be a set.

A synthetic basis on $S$ is a subset $\mathcal B \subseteq \mathcal P \left({S}\right)$ of the power set of $S$ such that:

\((B1)\)   $:$   $\mathcal B$ is a cover for $S$             
\((B2)\)   $:$     \(\displaystyle \forall U, V \in \mathcal B:\) $\exists \mathcal A \subseteq \mathcal B: U \cap V = \bigcup \mathcal A$             

That is, the intersection of any pair of elements of $\mathcal B$ is a union of sets of $\mathcal B$.