functoriality of categories of presheaves



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Given a small category CC, one can consider the category of presheaves PSh(C,D)PSh(C, D) valued in some category DD. Given some assumptions on DD, any functor of small categories F:CCF : C \to C' induces two adjoint pairs

F !:PSh(C,D)PSh(C,D):F * F_! : PSh(C, D) \rightleftarrows PSh(C', D) : F^*
F *:PSh(C,D)PSh(C,D):F * F^* : PSh(C', D) \rightleftarrows PSh(C, D) : F_*



Let F:CCF : C \to C' be a functor of small categories and DD some category. The restriction of scalars functor F *:PSh(C,D)PSh(C,D)F^* : PSh(C', D) \to PSh(C, D) is given by the formula HHfH \mapsto H \circ f, i.e. mapping a presheaf H:C opDH : C'^{op} \to D to the composite

C opFC opHD.C^{op} \stackrel{F}{\to} C'^{op} \stackrel{H}{\to} D.

Suppose DD admits small colimits (resp. small limits). Then the functor F *F^* admits a left adjoint F !F_! (resp. right adjoint F *F_*).



Let F:CC:GF : C \rightleftarrows C' : G be an adjoint pair and consider the induced functors (F !,F *,F *)(F_!, F^*, F_*) and (G !,G *,G *)(G_!, G^*, G_*). One has

  • F !F_! is left adjoint to G !G_!,
  • F *F^* is left adjoint to G *G^*,
  • F *F_* is left adjoint to G *G_*,
  • F *=G *F_* = G^*,
  • F *=G !F^* = G_!.

Note that all these claims are in fact equivalent.


If FF is fully faithful, then so are F !F_! and F *F_*.


A left adjoint functor LRL \dashv R is fully faithful precisely if RLR L is naturally isomorphic to the identity functor (by the unit). Dually, RR is fully faithful precisely if LRL R is naturally isomorphic to the identity (by the co-unit). Hence, it suffices to prove F *F !IdF^* F_! \cong Id, which by uniqueness of the right adjoint immediately implies that F *F *IdF^* F_* \cong Id and thus proves both claims.

Being a left adjoint, F *F !F^* F_! preserves colimits. Because every presheaf is a colimit of representable objects, it is sufficient to show that F *F !yyF^* F_! y \cong y where yy is the Yoneda-embedding. We have

(F *F !yI)J(F *yFI)J=(yFI)(FJ)=Hom(FJ,FI)Hom(J,I)=(yI)J. (F^* F_! y I) J \cong (F^* y F I) J = (y F I) (F J) = Hom(F J, F I) \cong Hom(J, I) = (y I) J.

See also

For functoriality of sheaves, see


Last revised on January 29, 2020 at 04:54:24. See the history of this page for a list of all contributions to it.