Topology axiom
= Topology axiom
A family $\mathcal T$ of <subset>[subsets] of a <set> $X$ is a topology when $\varnothing,X\in\mathcal T$, every <union> of members of $\mathcal T$ belongs to $\mathcal T$, and every finite <intersection> of members of $\mathcal T$ belongs to $\mathcal T$.