• Functoriality. The assignment $U\mapsto f_{!}\webleft (U\webright )$ defines a functor
    \[ f_{!}\colon \webleft (\mathcal{P}\webleft (X\webright ),\subset \webright )\to \webleft (\mathcal{P}\webleft (Y\webright ),\subset \webright ). \]

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

    • If $U\subset V$, then $f_{!}\webleft (U\webright )\subset f_{!}\webleft (V\webright )$.


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