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morphism of sites

Context

Topos Theory

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Morphisms of sites

Idea

A morphism of sites is, unsurprisingly, the appropriate sort of morphism between sites. It is defined exactly so as to induce a geometric morphism between toposes of sheaves (or, more generally, exact completions).

Definition

Let CC and DD be sites.

Definition

A functor f:CDf:C\to D is a morphism of sites if

  1. ff is covering-flat, and

  2. ff preserves covering families, i.e. for every covering {p i:U iU}\{p_i : U_i \to U\} of an object UCU \in C, the family {f(p i):f(U i)f(U)}\{f(p_i) : f(U_i) \to f(U)\} is a covering of f(U)Df(U) \in D.

Remark

If CC has finite limits and all covering families in DD are strong epic, then covering-flatness of ff is equivalent to ff preserving finite limits, i.e. being a left exact functor, or equivalently to being a representably flat functor. Thus, frequently in the literature one finds a definition of a morphism of sites as being representably flat and preserving covering families.

For general CC and DD, however, being representably flat implies being covering-flat, but not conversely. Thus, the above definition of morphism of sites is more general than the common one. There are few practical examples where the distinction matters, but our definition has better formal properties (see below).

Examples

Example

If AA and BB are frames regarded as sites via their canonical coverages, then a morphism of sites ABA \to B is equivalently a frame homomorphism, a function preserving finite meets and arbitrary joins.

Example

(slice sites)

For CC a site and UCU \in C, the comma category (C/U)(C / U) inherits a topology from CC, such that the forgetful functor (C/U)C(C/U) \to C constitutes a morphism of sites. This is also called the big site of UU. There are natural operations for restriction and extension of sheaves from a sub-site UU to XX and back.

For instance, if XX is a topological space and UOp(X)U \in Op(X) is an open subset, then UU regarded as a topological space in its own right has corresponding to it the site Op(U)=Op(X)UOp(U) = Op(X) \downarrow U.

Example

For CC and DD regular categories equipped with their regular coverages, a morphism of sites is the same as a regular functor, i.e. a functor preserving finite limits and covers.

More generally, if CC and DD are κ-ary regular categories with their κ\kappa-canonical topologies, then a morphism of sites is the same as a κ\kappa-ary regular functor (preserving finite limits and κ\kappa-ary effective-epic families).

Example

For CC any site with finite limits, there is canonically a morphism of sites to its tangent category. See there for details.

Properties

Relation to geometric morphisms

We discuss how morphisms of sites induce geometric morphisms of the corresponding sheaf toposes, and conversely. The reader might want to first have a look at the discussion of geometric morphisms – between presheaf toposes.

Let f:(𝒞,J)(𝒟,K)f : (\mathcal{C},J) \to (\mathcal{D},K) be a morphism of sites, with 𝒞\mathcal{C} and 𝒟\mathcal{D} small. Then precomposition with ff defines a functor between categories of presheaves ()f:PSh(𝒟)PSh(𝒞)(-)\circ f : PSh(\mathcal{D}) \to PSh(\mathcal{C}).

Proposition

There is a geometric morphism between the categories of sheaves

(f *f *):Sh(𝒟,K)f *f *Sh(𝒞,J) (f^* \dashv f_*) : Sh(\mathcal{D},K) \stackrel{\overset{f^*}{\leftarrow}}{\underset{f_*}{\to}} Sh(\mathcal{C},J)

where f *f_* is the restriction of ()f(-)\circ f to sheaves.

For the classical definition of morphisms of sites, using representably-flat functors, this appears for instance as (Johnstone, lemma C2.3.3, cor. C2.3.4). We give the proof in this special case; for the general case see (Shulman).

Proof

By the assumption that ff preserves covers we have that the restriction of ()f(-)\circ f to Sh K(𝒟)Sh_K(\mathcal{D}) indeed factors through Sh(𝒞)PSh(𝒞)Sh(\mathcal{C}) \hookrightarrow PSh(\mathcal{C}).

Because for {U iU}\{U_i \to U\} a cover in 𝒞\mathcal{C} and FF a sheaf on 𝒟\mathcal{D}, we have that (assuming here for simplicity that 𝒞\mathcal{C} has finite limits)

PSh 𝒞(lim( i,jU i UU j) iU i),F(f())) lim ( i,jPSh 𝒞(U i UU j,F(f())) iPSh C(U i,F(f()))) lim ( i,jF(f(U i UU j))) iF(f(U i))) lim ( i,jF(f(U i) f(U)f(U j)))) iF(f(U i))) PSh 𝒟(lim( i,jf(U i) f(U)f(U j)) if(U i)),F), \begin{aligned} PSh_{\mathcal{C}}( \underset{\to}{\lim} ( \coprod_{i, j} U_i \prod_{U} U_j) \stackrel{\to}{\to} \coprod_{i} U_i ) \;,\; F(f(-)) ) & \simeq \lim_{\leftarrow} \left( \prod_{i,j} PSh_{\mathcal{C}}( U_i \prod_U U_j, F(f(-))) \stackrel{\leftarrow}{\leftarrow} \prod_i PSh_C(U_i , F(f(-))) \right) \\ & \simeq \lim_{\leftarrow} \left( \prod_{i,j} F(f(U_i \prod_U U_j))) \stackrel{\leftarrow}{\leftarrow} \prod_i F(f(U_i)) \right) \\ & \simeq \lim_{\leftarrow} \left( \prod_{i,j} F(f(U_i) \prod_{f(U)} f(U_j)))) \stackrel{\leftarrow}{\leftarrow} \prod_i F(f(U_i)) \right) \\ & \simeq PSh_{\mathcal{D}}( \underset{\to}{\lim} ( \coprod_{i, j} f(U_i) \prod_{f(U)} f(U_j)) \stackrel{\to}{\to} \coprod_{i} f(U_i) ) \;,\; F ) \end{aligned} \,,

where we used the Yoneda lemma, the fact that the hom functor PSh(,)PSh(-,-) sends colimits in the first argument to limits, and the assumption that ff preserves the pullbacks involved.

Also ()f(-)\circ f preserves all limits, because for presheaves these are computed objectwise. And since the inclusion Sh K(𝒟)PSh(𝒟)Sh_K(\mathcal{D}) \to PSh(\mathcal{D}) is right adjoint (to sheafification) we have that

f *:Sh K(𝒟)PSh(𝒟)()fSh J(𝒞) f_* : Sh_K(\mathcal{D}) \hookrightarrow PSh(\mathcal{D}) \stackrel{(-)\circ f}{\to} Sh_J(\mathcal{C})

preserves all limits. Therefore by the adjoint functor theorem it has a left adjoint. Explicitly, this is the composite of the left adjoint to ()f(-)\circ f and to sheaf inclusion. The first is left Kan extension Lan fLan_f along ff and the second is sheafification L JL_J on (𝒞,J)(\mathcal{C},J), so the left adjoint is the composite

f *:Sh J(𝒞)PSh(𝒞)Lan fSPSh(𝒟)L JSh J(𝒟). f^* :Sh_J(\mathcal{C}) \hookrightarrow PSh(\mathcal{C}) \stackrel{Lan_f}{\to}S PSh(\mathcal{D}) \stackrel{L_J}{\to} Sh_J(\mathcal{D}) \,.

Here the first morphism preserves all limits, the last one all finite limits. Hence the composite preserves all finite limits if the left Kan extension Lan fLan_f does. This is the case if ff is a flat functor.

(Because the left Kan extension is given by the colimit Lan fX:dlim((f op/d)𝒞 opXSet)Lan_f X : d \mapsto {\underset{\to}{\lim}}((f^{op}/d) \to {\mathcal{C}}^{op} \stackrel{X}{\to} Set) over the comma category f op/df^{op}/d which is a filtered category if ff is flat, and filtered colimits are precisely those that commute with finite limits. For more details on this argument see the discussion at Geometric morphisms between presheaf toposes.)

Remark

This is a “contravariant” construction in that a morphism of sites going one way gives a geometric morphisms of toposes going the opposite way. Another condition, the covering lifting property gives a covariant assignment.

Conversely, any geometric morphism which restricts and corestricts to a functor between sites of definition is induced by a morphism between those sites:

Proposition

Let (𝒞,J)(\mathcal{C}, J) be a small site and let (𝒟,K)(\mathcal{D}, K) be a small-generated site. Then a geometric morphism

f:Sh(𝒟,K)Sh(𝒞,J) f : Sh(\mathcal{D}, K) \to Sh(\mathcal{C}, J)

is induced by a morphism of sites (𝒟,K)(𝒞,J):F(\mathcal{D}, K) \leftarrow (\mathcal{C}, J) : F precisely if the inverse image functor f *f^* respects the Yoneda embeddings, i.e. there is a functor FF making the following diagram commute:

𝒟 F 𝒞 j 𝒟 j 𝒞 Sh(𝒟,K) f * Sh(𝒞,J). \array{ \mathcal{D } &\stackrel{F}{\leftarrow}& \mathcal{C} \\ {}^{\mathllap{j_{\mathcal{D}}}}\downarrow && \downarrow^{\mathrlap{j_{\mathcal{C}}}} \\ Sh(\mathcal{D}, K) &\stackrel{f^*}{\leftarrow}& Sh(\mathcal{C}, J) } \,.

In the special case when 𝒞\mathcal{C} and 𝒟\mathcal{D} have finite limits and 𝒟\mathcal{D} is subcanonical, so that morphisms of sites can be defined using representably flat functors, this appears as (Johnstone, lemma C2.3.8). We give the proof in this case; for the general case see (Shulman, Prop. 11.14). Note that the general case would not be true for the classical definition of “morphism of sites”.

Proof

It suffices to show that given ff, the factorization FF is, if it exists, necessarily a morphism of sites: because since f *f^* is left adjoint and thus preserves all colimits and every object in Sh(C)Sh(C) is a colimit of representables, f *f^* is fixed by the factorization. By uniqueness of adjoint functors this means then that together with its right adjoint it is the geometric morphism induced from the morphism of sites, by prop. 1.

So we show that FF is necessarily a morphisms of sites:

  1. since the Yoneda embedding and sheafification as well as inverse images preserve finite limits, so does f *j 𝒞f^* j_{\mathcal{C}} and hence FF preserves finite limits, hence is a flat functor;

  2. f *h 𝒞f^* h_{\mathcal{C}} preserves coverings (maps them to epimorphisms in Sh(D,K)Sh(D, K)) and since KK is assumed to be subcanonical it follows from prop. \ref{CharacterizationOfSubcanonicalSites} that j 𝒟j_{\mathcal{D}} also reflects covers. Therefore FF preserves covers.

Corollary

Let (𝒞,J)(\mathcal{C},J) be a small site and let \mathcal{E} be any sheaf topos. Then we have an equivalence of categories

Topos(,Sh(𝒞,J))Site((𝒞,J),) Topos(\mathcal{E}, Sh(\mathcal{C}, J)) \simeq Site((\mathcal{C}, J), \mathcal{E})

between the geometric morphisms from \mathcal{E} to Sh(𝒞,J)Sh(\mathcal{C}, J) and the morphisms of sites from (𝒞,J)(\mathcal{C}, J) to the big site (,C)(\mathcal{E}, C) for CC the canonical coverage on \mathcal{E}.

This appears as (Johnstone, cor. C2.3.9).

Proof

Since for the canonical coverage the Yoneda embedding is the identity, this follows directly from prop. 2.

Remark

Corollary 1 leads to the notion of classifying toposes. See there for more details.

Between κ\kappa-ary sites

Proposition

If CC and DD are κ-ary sites, then a functor f:CDf:C\to D is a morphism of sites if and only if it preserves finite local κ\kappa-prelimits.

Proof

See (Shulman, Prop. 4.8)

References

Revised on October 9, 2014 15:48:45 by Adeel Khan (132.252.62.147)