• Functoriality. The assignment $\mathcal{U}\mapsto \bigcap _{U\in \mathcal{U}}U$ defines a functor
    \[ \bigcap \colon \webleft (\mathcal{P}\webleft (\mathcal{P}\webleft (X\webright )\webright ),\supset \webright )\to \webleft (\mathcal{P}\webleft (X\webright ),\subset \webright ). \]

    In particular, for each $\mathcal{U},\mathcal{V}\in \mathcal{P}\webleft (\mathcal{P}\webleft (X\webright )\webright )$, the following condition is satisfied:

    • If $\mathcal{U}\subset \mathcal{V}$, then $\displaystyle \bigcap _{V\in \mathcal{V}}V\subset \bigcap _{U\in \mathcal{U}}U$.


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