nLab
closed monoidal structure on presheaves

Context

Monoidal categories

Category theory

Contents

Idea

As every topos, the category PSh(X)PSh(X) of presheaves is cartesian closed monoidal.

Definition

Let SS be a category.

The standard monoidal structure on presheaves PSh:=[S op,Set]PSh := [S^{op}, Set] is the cartesian monoidal structure.

Recalling that limits of presheaves are computed objectwise, this is the pointwise cartesian product in Set: for two presheaves F,GF, G their product presheaf F×GF \times G is given by

F×G:UF(U)×G(U), F \times G : U \mapsto F(U) \times G(U) \,,

where on the right the product is in Set.

Proposition

The corresponding internal hom

[,]:PSh op×PShPSh [-,-] : PSh^{op} \times PSh \to PSh

exists and is given by

[F,G]:UHom PSh(Y(U)×F,G), [F,G] : U \mapsto Hom_{PSh}( Y(U)\times F, G ) \,,

where Y:S[S op,Set]Y : S \to [S^{op}, Set] is the Yoneda embedding.

Proof

First assume that [F,G][F,G] exists, so that by the hom-adjunction isomorphism we have Hom(R,[F,G])Hom(R×F,G)Hom(R, [F,G]) \simeq Hom(R \times F, G). In particular, for each representable functor R=Y(U)R = Y(U) (with YY the Yoneda embedding) and using the Yoneda lemma we get

[F,G](U) Hom(Y(U),[F,G]) Hom(Y(U)×F,G). \begin{aligned} [F,G](U) & \simeq Hom(Y(U), [F,G]) \\ & \simeq Hom(Y(U) \times F, G) \end{aligned} \,.

So if the internal hom exists, it has to be of the form given. It remains to show that with this definition [F,][F,-] really is right adjoint to F-\otimes F.

See (MacLane-Moerdijk, pages 46, 47).

Definition in terms of homs of direct images

Often another, equivalent, expression is used to express the internal hom of presheaves:

Let XX be a pre-site with underlying category S XS_X. Recall from the discussion at site that just means that we have a category S XS_X on which we consider presheaves FPSh(S X):=[S X op,Set]F \in PSh(S_X) := [S_X^{op}, Set], but that it suggests that

  • to each object UPSh(X)U \in PSh(X) and in particular to each US XPSh(X)U \in S_X \hookrightarrow PSh(X) there is naturally associated the pre-site UU with underlying category the comma category S U=(Y/Y(U))S_U = (Y/Y(U));

  • that the canonical forgetful functor j UX t:S US Xj^t_{U \to X} : S_U \to S_X, which can be thought of as a morphism of pre-sites j UX:XUj_{U \to X} : X \to U induces the direct image functor (j UX) *:PSh(X)PSh(U)(j_{U \to X})_* : PSh(X) \to PSh(U) which we write FF UF \mapsto F|_U.

Then in these terms the above internal hom for presheaves

hom:PSh(X) op×PSh(X)PSh(X) hom : PSh(X)^{op} \times PSh(X) \to PSh(X)

is expressed for all F,GPSh(X)F,G \in PSh(X) by

hom(F,G):UHom PSh(U)(F U,G U). hom(F,G) : U \mapsto Hom_{PSh(U)}(F|_U, G|_U) \,.

Relation of the two definitions

To see the equivalence of the two definitions, notice

  • that by the Yoneda lemma we have that S US_U is simply the over category S U=S X/US_U = S_X/U;
  • by the general properties of functors and comma categories there is an equivalence PSh(S X/U)PSh(S X)/y(U)PSh(S_X/U) \simeq PSh(S_X)/y(U);
  • which identifies the functor () U:PSh(S X)PSh(S U)(-)|_U : PSh(S_X) \to PSh(S_U) with the functor (()×y(U)p 2y(U)):PSh(S X)PSh(S X)/y(U)((-)\times y(U) \stackrel{p_2}{\to} y(U)) : PSh(S_X) \to PSh(S_X)/y(U);
  • and that Hom PSh(S X)/y(U)(y(U)×F,y(U)×G)Hom PSh(S X)(y(U)×F,G)Hom_{PSh(S_X)/y(U)}(y(U) \times F, y(U) \times G) \simeq Hom_{PSh(S_X)}(y(U) \times F, G).

References

The first definition is discussed for instance in section I.6 of

The second definition is discussed for instance in section 17.1 of

Revised on September 26, 2012 03:01:57 by Urs Schreiber (82.169.65.155)