Past exam of the mathematics course of the University of Cambridge / 2002 / iii / Paper 20 / 7 / ii

The two adjoints of inverse image on power sets areThese maps and $p^{\ast }(V)=p^{-1}(V)$ are monotone functions on the inclusion-ordered power sets. For $U\subseteq A$ and $V\subseteq B$,The first equivalence follows by following each element of $U$ through $p$. For the second, every fiber over $b\in V$ must lie in $U$, which is precisely the defining condition for $b\in {\forall }_p(U)$. These are the two Galois connections, proving ${\exists }_p⊣p^{\ast }⊣{\forall }_p$. In particular, a point outside $p(A)$ has an empty fiber and belongs to ${\forall }_p(U)$ even when $U$ is empty; omitting this case would give an incorrect right adjoint.