# Homology and Cohomology of Stacks (Lecture 7)

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1 Homology and Cohomology of Stacks (Lecture 7) February 19, 2014 In this course, we will need to discuss the l-adic homology and cohomology of algebro-geometric objects of a more general nature than algebraic varieties: for example, the moduli stack Bun G (X) of G-bundles on an algebraic curve. Let us therefore briefly review the language of stacks. Let Ring k denote the category of finitely generated k-algebras. If X is a k-scheme and R Ring k, then an R-valued point of X is a map Spec R X in the category of k-schemes. The collection of all R-valued points of X forms a set X(R). The construction R X(R) determines a functor from Ring k to the category of sets. We refer to this functor as the functor of points of X. If X is of finite type over k (or if we were to enlarge Ring k to include k-algebras which are not finitely generated), then X is determined by its functor of points up to canonical isomorphism. In this case, we will generally abuse notation by identifying X with its functor of points. Now suppose that G is a smooth affine group scheme over an algebraic curve X. We would like to introduce an algebro-geometric object Bun G (X) which classifies G-bundles on X. In other words, we would like the R-points of Bun G (X) to be G-bundles on the relative curve X R = Spec R Spec k X. Here some caution is in order. The collection of all G-bundles on X R naturally forms a category, rather than a set. Let us denote this category by Bun G (X)(R). If φ : R R is a k-algebra homomorphism, then φ determines a map of categories φ : Bun G (X)(R) Bun G (X)(R ), given on objects by the formula φ P = X R XR P. However, this construction is not strictly functorial: given another ring homomorphism ψ : R R, the iterated pullback ψ (φ P) = X R XR (X R XR P) is canonically isomorphic to X R XR P, but it is unnatural to expect that they should be literally identical to one another. It is possible to axiomatize the functorial behavior exhibited by the construction R Bun G (X)(R) using the language of 2-categories. However, it is often more convenient to encode the same data in a different package, where the functoriality is implicit rather than explicit. Definition 1. Let X be an algebraic curve over k and let G be a smooth group scheme over X. We define a category Bun G (X) as follows: (1) The objects of Bun G (X) are pairs (R, P), where R is a finitely presented k-algebra and P is a G-bundle on the relative curve X R = Spec R Spec k X. (2) A morphism from (R, P) to (R, P ) consists of a k-algebra homomorphism φ : R R together with a G-bundle isomorphism α between P and X R XR P. We will refer to Bun G (X) as the moduli stack of G-bundles. By construction, the construction (R, P) R determines a forgetful functor π : Bun G (X) Ring k. Moreover, for every finitely generated k-algebra R, we can recover the category Bun G (X)(R) of G-bundles 1

2 on X R as the fiber product Bun G (X) Ringk {R}. Moreover, the map π also encodes the functoriality of the construction R Bun G (X)(R): given an object (R, P) Bun G (X)(R) and a ring homomorphism φ : R R, we can choose any lift of φ to a morphism (φ, α) : (R, P) (R, P ) in Bun G (X). Such a lift then exhibits P as a fiber product X R XR P. More generally, for any functor π : C D and any object D D, let C D denote the fiber product C D {D}. We might then ask if C D depends functorially on D, in some sense. This requires an assumption on the functor π. Definition 2. Let π : C D be a functor between categories. We say that a morphism α : C C in C is π-cocartesian if, for every object C C, composition with α induces a bijection Hom C (C, C ) Hom C (C, C ) HomD (πc,πc ) Hom D (πc, πc ). We will say that π is an op-fibration (also called a Grothendieck op-fibration) if, for every object C C and every morphism α 0 : πc D in the category D, there exists an π-cocartesian morphism α : C D with α 0 = π(α). Let k be an algebraically closed field, and let Ring k denote the category of finitely generated k-algebras. We define a prestack to be a category C equipped with a Grothendieck op-fibration π : C Ring k. Remark 3. We will generally abuse notation by identifying a prestack π : C Ring k with its underlying category C and simply say that C is a prestack, or that π exhibits C as a prestack. Example 4. The forgetful functor Bun G (X) Ring k is an op-fibration, and therefore exhibits Bun G (X) as a prestack. Example 5. Let X be a k-scheme. We can associate to X a category C X, which we call the category of points of X. By definition, an object of C X is a pair (R, φ), where R is a finitely presented k-algebra and φ : Spec R X is a map of k-schemes. A morphism from (R, φ) to (R, φ ) is a k-algebra homomorphism ψ : R R for which the diagram Spec R φ Spec(ψ) φ Spec R X commutes. The construction (R, φ) R defines a forgetful functor C X Ring k, which exhibits C X as a prestack. Example 6 (Grothendieck Construction). Let D be a category, and let U be a functor from D to the category Cat of categories. We can define a new category D U as follows: (1) The objects of D U are pairs (D, u) where D is an object of D and u is an object of the category U(D). (2) A morphism from (D, u) to (D, u ) consists of a pair (φ, α), where φ : D D is a morphism in D and α : U(φ)(u) u is a morphism in U(D ). The construction (D, u) D determines a forgetful functor D U D which is a Grothendieck op-fibration. The passage from U to D U is often called the Grothendieck construction. For any Grothendieck op-fibration F : C D, the category C is equivalent to D U, for some functor U : D Cat. Moreover, the data of F and the data of the functor U are essentially equivalent to one another (in a suitable 2-categorical sense). 2

3 Remark 7. Let π : C Ring k and π : C Ring k be prestacks. A map of prestacks from C to C is a functor F : C C which carries π-cocartesian morphisms to π -cocartesian morphisms, and for which the diagram C F Ring k commutes. The collection of all maps of prestacks from C to C is organized into a category Hom(C, C ), where a morphism from F : C C to G : C C is a natural transformation of functors α : F G such that, for each object C C, the map π (α C ) is an identity morphism in Ring k. If π : C Ring k is another prestack, we have evident composition functors Hom(C, C ) Hom(C, C ) Hom(C, C ). We can summarize the situation by saying that the collection of all k-prestacks forms a (strict) 2-category. Remark 8. We can consider a more general notion of k-prestack morphism, given by a diagram C C C Ring k which commutes only up to specified isomorphism. The collection of all such morphisms can be organized into a category Hom (C, C ), which contains Hom(C, C ) as a subcategory. However, it is not hard to see that the inclusion Hom(C, C ) Hom (C, C ) is an equivalence of categories, so there is no real gain in generality. Remark 9. Let X and Y be k-schemes, and let Hom k (X, Y ) be the set of k-scheme maps from X to Y. We regard Hom k (X, Y ) as a category, having no morphisms other than the identities. We have an evident functor Hom k (X, Y ) Hom(C X, C Y ). If X is locally of finite type over k, then this map is an isomorphism of categories. In particular, the construction X C X determines a fully faithful embedding from the 1- category of schemes which are locally of finite type over k to the 2-category of prestacks. In other words, if X is a scheme which is locally of finite type over k, then X can be functorially reconstructed from the associated prestack C X. Because of this, we will generally abuse notation by identifying X with the prestack C X. Example 10. Let R be a finitely generated k-algebra, and let Spec R be the associated affine scheme. Then the prestack associated to Spec R is the category Ring R of finitely generated R-algebras (viewed as a prestack via the functor Ring R Ring k which forgets the R-algebra structure). Definition 11. Let π : C Ring k be a prestack. We say that C is a prestack in groupoids if every morphism of C is π-cocartesian. Remark 12. The condition that a prestack π : C Ring k be a prestack in groupoids is equivalent to the condition that for each R Ring k, the category C R = C Ringk {R} is a groupoid (that is, every morphism in C R is an equivalence). Example 13. All of the prestacks we have considered so far are prestacks in groupoids. For example, the prestack C Y associated to a k-scheme Y, the moduli stack Bun G (X), and the prestack Ran u G(X) are prestacks in groupoids. The prestack Ran(X) introduced in the previous lecture is not a prestack in groupoids. 3

4 Remark 14. In addition to the notion of prestack we have introduced, there is a more restrictive notion of stack. By definition, a stack (for the étale topology) is a prestack π : C Ring k for which the construction R C R satisfies descent (for the étale topology). Here we could replace the étale topology on Sch k by any other Grothendieck topology (Zariski, flat, Nisnevich, etcetera). All of the examples of prestacks that we have considered so far are stacks (for any of these topologies). Example 15. Let X be a quasi-projective k-scheme. We define a category Ran(X) as follows: The objects of Ran(X) are triples (R, S, µ) where R is a finitely generated k-algebra, S is a nonempty finite set, and µ : S X(R) is a map of sets. A morphism from (R, S, µ) to (R, S, µ ) in Ran(X) consists of a k-algebra homomorphism R R and a surjection of finite sets S S for which the diagram S S µ µ X(R) X(R ) commutes. The construction Ran(X) Ring k is an op-fibration, so we can regard Ran(X) as a prestack. We will refer to Ran(X) as the Ran space of X. More informally, Ran(X) is a prestack whose R-valued points are finite sets of R-valued points of X. Note that Ran(X) is not a prestack in groupoids (since a surjection of finite sets need not be bijective), and is not a stack for the étale topology on Sch k (or even the Zariski topology). Definition 16. Let l be a prime number which is invertible in k, and let Λ {Z l, Q l, Z/l d Z}. For any prestack π : C Ring k, we define chain complexes C (C; Λ) and C (C; Λ) by the formulae C (C; Λ) = lim C (Spec π(c); Λ) C C C (C; Λ) = lim C (Spec π(c); Λ). C C Here the limit and colimit are taken in the -category Mod Λ. We let H (C; Λ) denote the cohomology groups of C (C; Λ), and H (C; Λ) the homology groups of C (C; Λ). We refer to H (C; Λ) as the étale cohomology of C with coefficients in Λ, and H (C; Λ) as the étale homology of C with coefficients in Λ. Example 17. Let X Sch k be a quasi-projective k-scheme, and let C X be the associated prestack. If X = Spec R is affine, then the category C X has a final object (given by the pair (R, id)), so we have canonical equivalences C (C X ; Λ) C (Spec R; Λ) C (C X ; Λ) C (Spec R; Λ). By construction, the functor X C (X; Λ) satisfies descent for the étale topology. deduce the existence of general equivalences From this, one can C (C X ; Λ) C (X; Λ) C (C X ; Λ) C (X; Λ). Warning 18. Let C be a prestack. Then C (C; Λ) can be identified with the Λ-linear dual of C (C; Λ). In particular, if Λ is a field, then we have canonical isomorphisms H i (C; Λ) H i (C; Λ). However, C (C; Λ) need not be the Λ-linear dual of C (C; Λ) (if Λ is a field, this is true if and only if each H i (C; Λ) is a finite-dimensional vector space). 4

5 Warning 19. Let C be a prestack. Then C (C; Q l ) = C (C; Z l )[l 1 ], since the process of inverting l commutes with colimits. However, it generally does not commute with limits, so that the canonical map is not an equivalence in general. C (C; Z l )[l 1 ] C (C; Q l ) Warning 20. For every prestack C, we have a canonical equivalence C (C; Z l ) lim C (C; Z/l d Z). However, the canonical map C (C; Z l ) lim C (C; Z/l d Z) need not be an equivalence in general. Using Definition 16, we can precise formulate the main problem we will attempt to address in this course: Question 21. Let X be an algebraic curve over k and let G be a smooth affine group scheme over X. What can we say about the cohomology H (Bun G (X); Q l )? References [1] Freitag, E. and R. Kiehl. Etale cohomology and the Weil conjecture. Springer-Verlag [2] Laumon, G. and L. Moret-Bailly. Champs algébriques. Springer-Verlag,

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