Sometimes one finds the notation

\[ \forall _{f}\colon \mathcal{P}\webleft (X\webright )\to \mathcal{P}\webleft (Y\webright ) \]

for $f_{*}$. This notation comes from the fact that the following statements are equivalent, where $y\in Y$ and $U\in \mathcal{P}\webleft (X\webright )$:

  • We have $y\in \forall _{f}\webleft (U\webright )$.
  • For each $x\in X$, if $y=f\webleft (x\webright )$, then $x\in U$.
We will not make use of this notation in the present work.


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