Motivic Cohomology I: presheaves with transfers and sheaves with transfers

Where we’re at, and where we’re going

We last left off with: Motivic Cohomology I: correspondences. In this post it was described how we could construct a new category, \mathsf{Cor}_k, so that we have a faithful embedding \mathsf{Sm}_k\hookrightarrow \mathsf{Cor}_k of the category of smooth schemes over k into this newly defined category. The objects remained the same, but we now have correspondences as morphisms. It was observed \mathsf{Cor}_k is an additive category and, in addition, has a symmetric monoidal structure given by the tensor product.

In this post, we’ll construct an abelian category where \mathsf{Cor}_k embedds fully faithfully through the use of presheaves and Yoneda’s lemma. Afterwards, we extend the sheaf notion to our new presheaves for various Grothendieck topologies. We show our representable objects form sheaves in these respective topologies. We conclude by observing, for the étale and Nisnevich topologies, these presheaves admit a sheafification functor.

Presheaves With Transfers

We’ll start out with the definition:

(2.1) Definition: A presheaf with transfers is a contravariant additive functor from the category \mathsf{Cor}_k to the category of abelian groups \mathsf{Ab}.

The condition that a presheaf with transfers be an additive functor can be reformatted into conditions (C1) an (C2) below.

(C1) a presheaf with transfers is a contravariant functor taking sums to sums, taking summands with their inclusion into a sum to summands with their inclusion into a sum, and taking the zero object to the zero object.

(C2) for every smooth schemes X,Y and presheaf with transfers F, the map of sets \text{Cor}(X,Y)\rightarrow \text{Hom}(F(Y),F(X)) associated to the functor F is a homomorphism of abelian groups which commutes with the composition maps \text{Cor}(Y,Z)\otimes \text{Cor}(X,Y)\rightarrow \text{Cor}(X,Z) and \text{Hom}(F(Y),F(X))\otimes \text{Hom}(F(Z),F(Y))\rightarrow \text{Hom}(F(Z),F(X)).

Our interest will be in a new category, \mathsf{PreSh}(\mathsf{Cor}_k), whose objects are presheaves with transfers and whose morphisms are natural transformations between functors.

Remark: I’m not overly fond of the use of the term presheaf with transfers. The concept of a sheaf outdates the concept of a presheaf. The word sheaf (on a topological space) was used because it gives some intuitive feel for what a sheaf is (originally a sheaf was defined as the espace étale associated to a presheaf). The terminology “presheaf” implies, at least I interpret it as, information giving rise to a sheaf (on a topological space, via sheafification). The information contained in a presheaf is that of a contravariant functor from the category of open sets of some topological space to some other category. In our case, a presheaf with transfers is not going to give rise to a sheaf on a specific topological space. A presheaf with transfers is going to give rise to a sheaf on every smooth scheme defined over k. Maybe pluri-sheaf, or multi-sheaf, would have been more appropriate.

The additional terminology “with transfers” is essentially perpendicular to the above discussion. Recall we have a faithful embedding \mathsf{Sm}_k\hookrightarrow \mathsf{Cor}_k. If we have a presheaf with transfers F, in other words a contravariant additive functor F:\mathsf{Cor}_k\hookrightarrow \mathsf{Ab}, we can consider the composition \mathsf{Sm}_k\hookrightarrow\mathsf{Cor}_k\rightarrow \mathsf{Ab} as a restriction of a presheaf to the category of smooth schemes. That is to say, we have a forgetful functor

\omega: \mathsf{PreSh}(\mathsf{Cor}_k)\rightarrow \mathsf{PreSh}(\mathsf{Sm}_k)

and transfers are exactly the data which we forget.

This can be phrased another way. Since in \mathsf{Cor}_k we have \text{Hom}_{\mathsf{Cor}}(X,Y)=\text{Cor}(X,Y), any presheaf with transfers will associate to a given correspondence \gamma\in \text{Cor}(X,Y) a homomorphism of abelian groups like so

(X\xrightarrow{\gamma} Y) \rightsquigarrow (F(Y)\xrightarrow{F(\gamma)}F(X)).

This association induces a map on sets \text{Cor}(X,Y)\times F(Y)\rightarrow F(X). Condition (C2) then guarantees this induces a homomorphism of abelian groups \text{Cor}(X,Y)\otimes F(Y)\rightarrow F(X).  Transfers are, in this wording, those maps associated to correspondences which do not arise as the graphs of honest morphisms f:X\rightarrow Y (my slang term honest means a morphism in the category \mathsf{Sm}_k).

 If this is your first time working with categories of functors (it probably isn’t) then you may be asking why are we considering a new category. Again. The answer is simple, this one is abelian! It isn’t that difficult to show this (or we could refer to 1.6.4 in [Wei]) and the proof comes down to defining the kernel and cokernel of a morphism of presheaves with transfers, say F\xrightarrow{\phi} G. But, we can just define the kernel to be, for every object X, \ker\phi(X):= \ker(F(X)\xrightarrow{\phi(X)} G(X)) and similarly define the cokernel replacing \ker by \text{coker}.

What is less obvious is knowing that \mathsf{Ab} having enough inejctives and projectives implies

(2.2) Proposition: The category \mathsf{PreSh}(\mathsf{Cor})_k has enough injectives and projectives.

Reference. [Wei] exercises 2.3.7, 2.3.8.

Essentially, we can theoretically compute cohomology (I say theoretically compute but, a better phrase is probably “there exists a satisfactory cohomology theory satisfying various functorial properties”) for objects in the category \mathsf{PreSh}(\mathsf{Cor}_k).

For now, we’ll leave this category here, because we’ll actually be using a different category, constructed from \mathsf{PreSh}(\mathsf{Cor}_k) by applying some category-theoretic-tools which I will cover in my next post. However, we’ll point out that Yoneda’s lemma shows

(2.3) Proposition: There is a fully faithful embedding \mathsf{Cor_k}\hookrightarrow \mathsf{PreSh}(\mathsf{Cor}_k).

If X is a smooth scheme, then the embedding takes X to the functor \mathbb{Z}_{\text{tr}}X:=\text{Hom}_{\mathsf{Cor}}(-,X)=\text{Cor}(-,X), or the functor which assigns to any scheme Y the abelian group \text{Cor}(Y,X). (We’ll use \mathbb{Z}_{\text{tr}}(X) and \mathbb{Z}_{\text{tr}}X interchangably). The functors \mathbb{Z}_{\text{tr}}X are said to be represented by X. In the R-linear category \mathsf{Cor}_k(R) we’ll call the functor represented by X (when X is considered as an object in \mathsf{Cor}_k(R)) R_{\text{tr}}X instead of \mathbb{Z}_{\text{tr}}X. We can extend all of the above definitions to the category \mathsf{Cor}_k(R). We write \mathsf{PreSh}(\mathsf{Cor}_k(R)) for the category of presheaves F:\mathsf{Cor}_k(R)\rightarrow R-mod.

Remark: We can assign a similar functor to any scheme Y smooth or not. That is, we can also define \mathbb{Z}_{\text{tr}}Y:=\text{Cor}(-,Y). However, we can’t say much else about this functor.

Sheaves with transfers

Now let us specialize to the concept of sheaves with transfers. To have a meaningful discussion of these objects we will need the notion of a Grothendieck topology, \mathsf{C}_{\tau}. There are three topologies we focus on but, we can use any topology which satisfies the following condition:

(T1) For any smooth scheme X, the category \mathsf{C}_{\tau}(X) consisting of covers of X and allowed morphisms of covers includes naturally into \mathsf{Sch}_k (schemes over k). We require the the inclusion factors \mathsf{C}_{\tau}(X)\hookrightarrow \mathsf{Sm}_k\hookrightarrow \mathsf{Sch}_k.

What I want from this condition is that all of our covers are actually smooth schemes as well. This way we can use the information we have about the topology \mathsf{C}_{\tau}(X) for a fixed X without having to reprove it for just smooth covers. In the three topologies we focus (Zariski, Nisnevich, and Étale) this won’t be a problem since:

(2.4) Proposition: Let X be a smooth scheme. Any topology which occurs as a subcategory of \mathsf{Et}(X), the étale topology on X, satisfies condition (T1).

Proof. Since étale morphisms are smooth, the composition of any cover U\rightarrow X with the smooth structure map X\rightarrow \text{Spec}(k) is smooth.\square

We can expand the sheaf condition from one scheme to \mathsf{Sm}_k with:

(2.5) Definition: Let \mathsf{C}_{\tau} be a Grothendieck topology satisfying (T1). A presheaf of abelian groups F:\mathsf{Sm}_k\rightarrow \mathsf{Ab} is a sheaf for the \mathsf{C}_{\tau} topology if the restriction to \mathsf{C}_{\tau}(X) is a sheaf for every smooth scheme X.

This is equivalent to the restriction satisfying exactness of the usual sequence 0\rightarrow F(X)\rightarrow \prod_i F(U_i)\rightarrow \prod_{i,j}F(U_i\times_X U_j) for every cover \{U_i\rightarrow X\} and F(U\amalg V)=F(U)\oplus F(V). In the cases we are concerned with (Zariski, étale, and Nishnevich topologies), it suffices to consider only finite indexing sets (that is, we need only consider covers which consist of finitely many covering maps). This is because, for a Noetherian site, there is a natural equivalence between the categories which contain only finite covering maps and the general case (a Noetherian site is one for which any cover has a finite subcover – in our case this follows from the fact étale maps are open, and any smooth scheme being finite type over k is quasi-compact). For this result, which we use in the proof of proposition 2.7, we refer to [Mil], Chapter 3 section 3 proposition 3.5 page 112.

We extend the definition to \mathsf{PreSh}(\mathsf{Cor}_k) similarly:

(2.6) Definition: A presheaf with transfers will be called a sheaf with transfers for the \mathsf{C}_{\tau} topology, if it’s restriction to \mathsf{Sm}_k is a sheaf for the \mathsf{C}_{\tau} topology.

The subcategories consisting of Zariski, étale, and Nisnevich sheaves with transfers will be denoted \mathsf{Sh}_{Zar}(\mathsf{Cor}_k), \mathsf{Sh}_{et}(\mathsf{Cor})_k, and \mathsf{Sh}_{Nis}(\mathsf{Cor}_k) respectively. For the most part, we’ll prove results in the category of sheaves for the étale and Nisnevich topologies (although we may have to digress to the cdh topology, among others, if we want to extend our definition of motives to all schemes). The Zariski topology will be our place to define most objects, like motivic cohomology. We’ll use the results we obtain in these finer topologies to prove more general results which we then specialize to the Zariski topology.

(2.7) Propsoition: For any scheme Y, the presheaf with transfers \mathbb{Z}_{\text{tr}}Y is a sheaf with transfers for the Zariski, étale, and Nisnevich topologies.

Proof. We’ll check exactness of the sequence

0\rightarrow \mathbb{Z}_{\text{tr}}Y(X)\rightarrow\prod_i \mathbb{Z}_{\text{tr}}Y(U_i)\rightarrow \prod_{i,j} \mathbb{Z}_{\text{tr}}Y(U_i\times_X U_j)

for any cover \{p_i:U_i\rightarrow X\}_i.

Injectivity of  \mathbb{Z}_{\text{tr}}Y(X)\rightarrow \prod_i \mathbb{Z}_{\text{tr}}Y(U_i):  Unraveling definitions we need to show \text{Cor}(X,Y)\xrightarrow{\Gamma_{p_i}} \prod_i \text{Cor}(U_i,Y) is injective. Let Z(A\times B) be the group of all cycles in the product A\times B. By definition we have inclusions \text{Cor}(A,B)\hookrightarrow Z(A\times B). For the étale (and hence Zariski and Nisnevich) topology, the map U_i\times Y\rightarrow X\times Y is flat and from intersection theory we know the flat pullback of cycles induces an injective map Z(X\times Y)\hookrightarrow \prod_i Z(U_i\times Y) (to see the injectivity, note that the map is given by taking a cycle to its preimage; since the cover is required to surject onto X, every cycle has a preimage whose image is itself). Thus, the result follows if we can show the following diagram commutes:

\begin{matrix} \text{Cor}(X,Y) & \rightarrow & \prod_i \text{Cor}(U_i,Y) \\ \downarrow & & \downarrow \\ Z(X\times Y)& \hookrightarrow & \prod_i Z(U_i\times Y) \end{matrix}

where the top horizontal arrow is composition (-\circ \Gamma_{p_i}), the bottom horizontal arrow is flat pullback of cycles, and the vertical arrows are the inclusions. These agree by construction.

Remark: For the Zariski topology one can make this more geometric: assume X is connected. Then any elementary correspondence W is surjective over X and, since X is smooth, W is irreducible. This implies the generic fiber of the projection W\subset X\times Y\rightarrow X is dense in W. Hence if we have two correspondences W,V which agree on U\times Y for some dense open U\subset X then, as U is an open subset of an irreducible scheme, it contains the generic point of X. Hence, both W,V are closed integral subschemes agreeing on a dense open subset of X\times Y and are therefore equal on all of X\times Y. Note also that this implies if U\subset X then the restriction \text{Cor}(X,Y)\rightarrow \text{Cor}(U,Y) is injective.

Following the definitions of the maps \text{Cor}(X,Y)\rightarrow \prod_i \text{Cor}(U_i,Y)\rightarrow \prod_{i,j} \text{Cor}(U_i \times_X U_j,Y), it’s clear this is a complex since

\gamma\mapsto (\gamma\circ \Gamma_{p_i})_i\mapsto \gamma\circ\Gamma_{p_i}\circ \Gamma_{\text{pr}_i}-\gamma\circ\Gamma_{p_j}\circ\Gamma_{\text{pr}_j}=\gamma\circ ( \Gamma_{p_i}\circ \Gamma_{\text{pr}_i} - \Gamma_{p_j}\circ \Gamma_{\text{pr}_j}) = \gamma\circ 0 = 0.

Here we used the fact that we have equality of maps U_i\times_X U_j\rightarrow X p_j\circ \text{pr}_j = p_i\circ \text{pr}_i by the construction of the fibered product.

Exactness in the middle: suppose (f_k)_k\mapsto 0 \in \prod_{i,j}\text{Cor}(U_i\times_X U_j, Y). We’ll invoke the proposition of Milne mentioned earlier to assume k is finite, or that our covering was finite all along (since we haven’t used it until this point). This lets us prove the proposition by working with only two indices 1,2.

So assume we have a pair (f_1,f_2)\in \text{Cor}(U_1,Y)\times \text{Cor}(U_2,Y) so that f_1|_{1,2} -f_2|_{1,2}=0\in \text{Cor}(U_1\times_X U_2, Y). We want to show that (f_1,f_2) is in the image of \text{Cor}(X,Y). Essentially what we need to do is find a way to extend correspondences on the “intersection” U_1\times_X U_2 to U_1,U_2. We can do this by using a “generic” argument or, by reducing the proof to the case X is field.

To do this reduction, we first extend everything to the function field, call it F, of X (if X isn’t irreducible, then we first replace it by an irreducible component and then argue by additivity of \text{Cor}(-,Y)). Next, we take limits over Zariski open covers of X_F. Since U_{1,F},U_{2,F} are étale over X_F their restrictions over the opens of X_F are as well. This means, in the limit, we are left with two étale covers of F which we know to be just a product of finite separable extensions of F. Call L_1 and L_2 the product of fields we get in the inverse limits of U_{1,F} and U_{2,F} respectively.  If we’re able to show the proposition when X is a field, then we’ve shown the proposition for the sequence

0\rightarrow \text{Cor}(F,Y_F)\rightarrow \text{Cor}(L_1,Y_F)\times \text{Cor}(L_2,Y_F)\rightarrow \text{Cor}(L_1\times_F L_2, Y_F).

We proceed by a diagram chase using the commutative diagram below.

\begin{matrix} 0 & \rightarrow & \text{Cor}(X,Y) & \rightarrow & \text{Cor}(U_1,Y)\times \text{Cor}(U_2,Y) & \rightarrow & \text{Cor}(U_1\times_X U_2) \\ & & \downarrow & & \downarrow & & \downarrow \\ 0 & \rightarrow & \text{Cor}(F,Y_F) & \rightarrow & \text{Cor}(L_1,Y_F)\times \text{Cor}(L_2,Y_F) & \rightarrow & \text{Cor}(L_1\times_F L_2, Y_F) \end{matrix}

The bottom row of this diagram is exact and all of the vertical arrows in this diagram are injective by the remark above. So, to show the claim we could try to verify the map \text{Cor}(X,Y)\rightarrow \text{Cor}(F,Y_F) is surjective. But this is not true! (For example, the hyperbola defines a finite correspondence in \text{Cor}(\mathbb{G}_m,\mathbb{A}^1) that can’t come from a finite correspondence in \text{Cor}(\mathbb{A}^1,\mathbb{A}^1). In other words, the inclusions \text{Cor}(\mathbb{A}^1,\mathbb{A}^1)\subset \text{Cor}(\mathbb{G}_m,\mathbb{A}^1)\subset \text{Cor}(k(t),\mathbb{A}^1_{k(t)}) are strict). Instead, from the exactness of the bottom row in the above diagram, we get a Zariski open subset W\subset X so that the sequence restricted to W is exact:

0\rightarrow \text{Cor}(W,Y) \rightarrow \text{Cor}(U_1\times_X W, Y) \times \text{Cor}(U_2\times_X W, Y)\rightarrow \text{Cor}(U_1\times_X W \times_X U_2\times_X W,Y).

In particular, we find a correspondence g \in \text{Cor}(W,Y) so that g\mapsto (f_1,f_2) under the second arrow from the left. But this is good enough: we can assume g, f_1, f_2 are elementary correspondences (or we can work with each summand) to argue that, since  \Gamma_{p_1}\circ g |_{U_1\times W} = f_1|_{U_1\times W} that in fact there is an equality between the closure of the restriction of g, \overline{\Gamma_{p_1}\circ g}, and f_1. The same argument works for f_2, which proves the proposition (modulo the next paragraph).

It remains to prove the proposition when X is a field. So we can assume also that U_1,U_2 are a product of finite separable extensions of X=\text{Spec}(F). We can assume further U_1,U_2 are fields themselves (by the additivity of the \text{Cor}(-,Y) functor) and that U_1=U_2=\text{Spec}(E) for some Galois field E by extending to their composite and then a Galois closure. By the primitive element theorem we can write E\otimes_F E\cong E\otimes _F F[x]/(p)\cong E[x]/(p) for some irreducible F-polynomial p which completely splits over E. Factoring p over E shows our correspondences restrict to a disjoint union of finitely many \text{Spec}(E) indexed by elements of the Galois group \text{Gal}(E/F). Now, if we have a pair (f_1,f_2) \in \text{Cor}(\text{Spec}(E),Y)\times \text{Cor}(\text{Spec}(E),Y) which is mapped to 0\in \text{Cor}(\text{Spec}(E\otimes_F E),Y) then f_1=\sigma(f_2) for every \sigma\in\text{Gal}(E/F) or, in other words, (f_1,f_2) is invariant under \text{Gal}(E/F). But this means exactly that it comes from an element of \text{Cor}(X,Y)=\text{Cor}(\text{Spec}(F),Y) which completes the proof. \square

For a cover p:U\rightarrow X we can construct a complex, the Čech complex, and denote it \mathbb{Z}_{\text{tr}}(\check{U}). In both the étale and Nisnevich topologies this complex is, in fact, a resolution by sheaves in the respective topology. In the Zariski topology this fails, and we’ll have to replace it by something different. Eventually we’ll show how to do this but, for now we focus on doing it for the cases present and, after learning why we were doing it in the first place some posts later, we’ll come back to the complicated Zariski discussion.

(2.8) Proposition: The Čech complex associated to a covering map p:U\rightarrow X (for example any finite cover \{U_i\rightarrow X\}_i gives rise to such a covering map in the étale and Nisnevich topologies by taking U to be the disjoint union of the U_i) is defined as the complex

\cdots \xrightarrow{\Gamma_0 -\Gamma_1 + \Gamma_2}\mathbb{Z}_{\text{tr}}(U\times_X U)\xrightarrow{\Gamma_0 - \Gamma_1} \mathbb{Z}_{\text{tr}}U \xrightarrow{\Gamma_p} \mathbb{Z}_{\text{tr}}X \rightarrow 0.

Here the correspondence \Gamma_i : U\times_X U\times_X \cdots \times_X U\rightarrow U\times_X \cdots \times_X U is the graph of the morphism deleting the ith component. The Čech complex is a resolution of \mathbb{Z}_{\text{tr}}X as an étale/Nisnevich sheaf by étale/Nisnevich sheaves respectively.

Proof. Showing this is a complex amounts to showing equality of maps U^{\times_X n+1} \rightarrow U^{\times_X n}\rightarrow U^{\times_X n-1} in the category \mathsf{Sm}_k. As these are presheaves, we just need to compute the composition of two terms starting from an arbitrary scheme Y and a correspondence \gamma\in \text{Cor}(Y,U^{\times n+1}). Explicitly our maps are:

\gamma \mapsto (\sum_{k=0}^n (-1)^k \Gamma_{k} )\circ \gamma \mapsto \sum_{j=0}^{n-1}(-1)^j \Gamma_j \circ ((\sum_{k=0}^n (-1)^k \Gamma_{k} )\circ \gamma).

What we can do is: see how this double sum behaves on points (x_0,\ldots,x_n), using the notation (x_0,\ldots,\hat{x}_i,\ldots, x_n) to represent the map \Gamma_i. If we did this, we’d find the same cancellation that occurs in the proof that, say, the boundary map in singular homology is actually a differential.

Remark: We can do this because these maps actually behave this way for R-valued points, in the functor language.

To see the comlpex has no homology, it suffices, since the objects of this complex are sheaves by Proposition 2.7 above, to compute homology in the category of sheaves. That is to say, we can check this complex is exact at the level of stalks. Or, for every smooth scheme S and every p\in S, the morphism will be exact if it is when evaluated on \mathcal{O}_{S,p}^{sh} in the étale topology, and \mathcal{O}_{S,p}^h in the Nisnevich topology. The idea will be to first rewrite the complex in a different, but equivalent, form and then use Hensel’s lemma to define a chain homotopy between the identity on this complex and the 0-map on this complex, which will prove the theorem by general homological algebra. The actual difficulty is finding a way to employ Hensel’s lemma (and of course checking that we can choose lifts up to homotopy).

Unfortunately, we need to introduce some notation for clarity. Let X be as above, let S be an arbitrary smooth scheme, and let Z be a closed subscheme of S\times X which is finite and surjective over S. L(Z,S\times X) denote the free abelian group of closed irreducible subschemes of Z which are finite and surjective over S. To clarify how to think about this, note if Z=S\times X then there’s an equality L(Z,S\times X)=\text{Cor}(S,X) as groups; in general we have an equality \varinjlim_{Z} L(Z,S\times X)= \text{Cor}(S,X) where the limit runs over all closed subschemes Z\subset S\times X. Using this, we can rewrite our complex as a direct limit of complexes:

\begin{matrix} \cdots \rightarrow \mathbb{Z}_{tr}(U\times_X U) \rightarrow \mathbb{Z}_{tr}U\rightarrow \mathbb{Z}_{tr}X\rightarrow 0 \\ \simeq \\ \mathscr{L}^\bullet:= \cdots \rightarrow \varinjlim_{Z} L(Z\times_X\times U^{\times_X 2} -\times U^{\times_X 2})\rightarrow \varinjlim_{Z} L(Z\times_X U, -\times U) \rightarrow \varinjlim_{Z} L(Z,-\times X)\rightarrow 0 \end{matrix}

The utility in doing this is that it’s possible to show, given a closed subscheme Z\subset S\times X finite and surjective over S, that we can define a map \mathscr{L}^\bullet\rightarrow \mathscr{L}^{\bullet+1} whenever S is a Hensel local scheme (which we can also assume, since we’re working stalkwise). To see this, note that since S is a Hensel local scheme and Z is finite over S, then Z is also a Hensel local scheme. By [The Stacks Project, Lemma 10.148.3], from the projection map \pi_1:Z\times_X U\rightarrow Z there is a splitting s_1:Z\rightarrow Z\times_X U (i.e. a map with \pi_1\circ s_1 = \text{id}_Z). From this section we get maps s_k:=s_1\times \text{id}_{Z_U^{\times k}}: Z\times U^{\times k-1}=Z\times_Z(Z\times U)^{\times k-1} \rightarrow (Z\times U)^{k}= Z\times U^{\times k}. We define the chain homotopy f_k:\mathscr{L}^k \rightarrow \mathscr{L}^{k +1} by composing with \Gamma_{s_k} as correspondences.

We still need to check that (\sum_{i=0}^k (-1)^i \Gamma_i) \circ \Gamma_{s_k}+\Gamma_{s_{k-1}} \circ (\sum_{i=0}^{k-1} (-1)^i \Gamma_i) = \text{id}. In simple cases I can check this but I don’t know a general approach right now. (It can probably be done formally, in the same way we can check most of the other claims about compositions in this field). \square

Finally, we arrive at our last result for this section which concerns sheafification in the various topologies. Let \mathsf{C}_\tau be a Grothendieck topology satisfying (T1).  Let the inclusion of the subcategory of sheaves for this topology into the category of presheaves be denoted \iota :\mathsf{Sh}_{\mathsf{C}_\tau}(\mathsf{Sm}_k)\rightarrow \mathsf{PreSh}(\mathsf{Sm}_k). When there is an adjoint a_{\tau}:\mathsf{PreSh}(\mathsf{Sm}_k)\rightarrow \mathsf{Sh}_{\mathsf{C}_\tau}(\mathsf{Sm}_k) we say that \mathsf{C}_\tau admits sheafification.

When \mathsf{C}_\tau is either the étale or the Nisnevich topologies, it is known there exists a sheafification. We’ll denote this map a_{et} and a_{Nis} respectively. For any presheaf with transfers F, we can restrict to a presheaf on \mathsf{PreSh}(\mathsf{Sm}_k) via \omega, and then sheafify with a_{et} or a_{Nis}. Let F^s_{et} or F^s_{Nis} be the sheaf on \mathsf{Sm}_k that we get after applying this procedure.

(2.9) Proposition: There is a presheaf with transfers F_{et} (or F_{Nis} for the Nisnevich topology) so that any morphism F\rightarrow F^s_{et} (resp. F\rightarrow F^s_{Nis}) as presheaves on \mathsf{Sm}_k factors uniquely through F\rightarrow F_{et}\rightarrow F_{et}^s (resp. F\rightarrow F_{Nis}\rightarrow F_{Nis}^s) with the first map being a morphism of presheaves with transfers and the latter a morphism of sheaves on the category \mathsf{Sm}_k.

Reference: [MVW] Theorem 6.17, page 42 in the étale case; Theorem 13.1, page 99 for the Nisnevich.

Remark: Again this proposition fails for the Zariski topology but, we can fix this by only considering presheaves with transfers satisfying additional conditions. We’ll take this approach in a later post.

Some arguments I won’t go into details about allow us to conclude

(2.10) Proposition: the categories \mathsf{Sh}_{et}(\mathsf{Cor}_k) and \mathsf{Sh}_{Nis}(\mathsf{Cor}_k) have enough injectives.

Reference: Proposition 13.1 in [MVW] for the Nisnevich case which really just says the proof in the étale case, Proposition 6.19 in [MVW], carries through to the Nisnevich topology as well.

Tensor Products and Internal Homs of Presheaves with Transfers

Our construction of the tensor product for presheaves with transfers is going to extend our definition of the tensor product in the category \mathsf{Cor}_k. Recall we defined, for any smooth schemes X,Y over k, X\otimes Y :=X\times_k Y. We can, in fact, do this construction with R coefficients for some ring R.

Recall the Yoneda embedding

\mathsf{Cor}_k(R)\hookrightarrow \mathsf{PreSh}(\mathsf{Cor}_k(R))

defined on objects as X\mapsto \text{Hom}_{\mathsf{Cor}(R)}(-,X)=: h_X. Using the Yoneda lemma this immediately gives:

(2.11) Lemma: The representable objects h_X are projective objects of \mathsf{PreSh}(\mathsf{Cor}_k(R)).

Proof. The Yoneda lemma says \text{Hom}_{\mathsf{PreSh}(\mathsf{Cor}(R))}(h_X, F)\cong F(X) for an arbitrary functor F. Hence, if we have an exact sequence of presheaves

0\rightarrow F\rightarrow G\rightarrow H\rightarrow 0


0\rightarrow \text{Hom}(h_X,F)\rightarrow \text{Hom}(h_X,G)\rightarrow \text{Hom}(h_X,H)\rightarrow 0

is exact since it is canonically the sequence

0\rightarrow F(X)\rightarrow G(X)\rightarrow H(X)\rightarrow 0

which is exact by assumption.\square.

From (2.11) we can also deduce

(2.12) Lemma: Every presheaf F has a projective resolution. That is, \mathsf{PreSh}(\mathsf{Cor}_k(R)) has enough projectives.

Proof. This is a result of the natural surjection, for arbitrary presheaves F,

\bigoplus_{X\in \text{obj}(\mathsf{Cor}_k(R))}\bigoplus_{x\in F(X)}h_X\xrightarrow{x} F.

From here the resolution can be constructed inductively, at each step having the natural surjection onto the kernel of the previous map. \square

Remark: In the above, it’s implicitly used that the objects of \mathsf{Cor}_k(R) form a set instead of a proper class.

Lemma (2.11) and (2.12) have the benefit of allowing us to define a tensor product structure extending the one we’ve already defined on \mathsf{Cor}_k(R). Note, if we try to naively define (F\otimes_R G) (X)= F(X)\otimes_R G(X) then we lose additivity (i.e. for a smooth connected scheme X we would have (F\otimes_R G)(X\coprod X)\simeq (F(X)\otimes_R G(X))^{\oplus 4} which is not generally equal to (F(X)\otimes_R G(X))^{\oplus 2}) , hence this is not a presheaf with transfers.

Instead, we proceed as follows.

(2.13) Construction: From the Yoneda embedding \mathsf{Cor}_k(R)\hookrightarrow \mathsf{PreSh}(\mathsf{Cor}_k(R)) we can define a product \otimes on projective objects: h_X\otimes h_Y:=h_{X\otimes Y} which has the same formal properties as \otimes on \mathsf{Cor}_k(R).

We extend the definition of \otimes by defining, for arbitrary direct sums (\oplus_i h_{X_i})\otimes h_Y=\oplus_i h_{X_i\otimes Y}.

Now let F,G be arbitrary presheaves with transfers, P_\ast\rightarrow F and Q_\ast\rightarrow G projective resolutions as given in (2.12). The total complex \text{Tot}(P_\ast \otimes Q_\ast) is defined by the previous two pargraphs.

(2.14) Definition: We write F\otimes_L G for the total complex \text{Tot}(P_\ast \otimes Q_\ast). The tensor product of presheaves with transfers and internal Hom presheaf with transfers are defined:

F\otimes^{tr} G= H_0(F\otimes_L G)

\underline{\text{Hom}}(F,G): X\mapsto \text{Hom}(F\otimes h_X, G).

Remark: These are well-defined up to chain homotopy equivalence since any projective resolutions of the same functor are homotopy equivalent.

(2.14) Corollary: h_X\otimes_L h_Y=h_X\otimes h_Y = h_{X\otimes Y}.

Proof. Since h_X, h_Y are projective, they form a projective resolution of length 1. The first equality is then clear. The last equality follows from the way we extended the product via the Yoneda embedding (2.13).\square

The internal Hom and tensor product also satisfy expected exactness properties.

(2.15) Lemma: The functor \underline{\text{Hom}}(F,-) is right adjoint to F\otimes - . Hence, \underline{\text{Hom}}(F,-) is left exact and F\otimes - is right exact.

Proof. We have

\text{Hom}(h_X,\underline{\text{Hom}}(h_Y, G)) = G( X\otimes Y) = \text{Hom} (h_X\otimes h_Y, G).\square


[Mil] Etale Cohomology – James Milne. Not to be confused with his later work, “Lectures on Etale Cohomology”. Princeton Mathematical Series.

[MVW] Motivic Cohomology – Mazza, Voevodsky, Weibel. Link. (Try Google if the link expires).

[Wei] An Introduction to Homological Algebra – Weibel. Link. Cambridge Studies in Advanced Mathematics. Or Google it.


3 comments on “Motivic Cohomology I: presheaves with transfers and sheaves with transfers

  1. […] the last post, Motivic Cohomology I: presheaves with transfers and sheaves with transfers, we developed the notion of a presheaf with transfers. These were contravariant additive […]


  2. […] where the last two equalities ( ) follow from a computation similar to checking the boundary operator in singular homology is a differential (I said the same thing in the second post I wrote in this series for a different complex). […]


  3. […] following diagram shows the situation we are in, with the middle row obtained by applying the  Čech complex from Proposition (2.8) to the cover and of ; the objects on the bottom rows are the cokernels of the vertical […]


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