Lecture 9  Faithfully Flat Descent


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1 Lecture 9  Faithfully Flat Descent October 15, Descent of morphisms In this lecture we study the concept of faithfully flat descent, which is the notion that to obtain an object on a scheme X, it is enough to give an object on a faithfully flat cover Y of X, together with gluing or descent data. As this is best illustrated by example, we begin by studying descent of morphisms: Consider a scheme X, and an open covering (U i ) i I of X. Now for any scheme Z, to give a morphism φ : X Z it is equivalent to give morphisms φ i : U i Z which agree on intersections, which is to say that φ i U i U j = φ j U i U j. Noting that in the category of sets, fiber product is the same as intersection, our first theorem is a generalization of this fact to faithfully flat coverings; Theorem 1.1. Lt (U i X) i I be a covering in X fl, and Z a scheme. Suppose we have morphisms φ i : U i Z such that for all i, j I we have φ i U i X U j = φ j U i U j. Then there exists a unique map φ : X Z such that φ U i = φ i for all i I. Before we prove this theorem, we shall need the following lemma, which is a key ingredient in main incarnations of faithfully flat descent. Lemma 1.2. Let φ : A B be a faithfuly flat map of rings. Then ( ) 0 A φ B d 0 B A B is an exact sequence of Amodules, where d 0 (b) := b 1 1 b. 1
2 Proof. Since φ is injective, we consider A as a subset of B. We proceed in three stages: 1. Suppose that there exists a section g : B A such that g A = Id A. Then we can write B = A I where I = ker g is a Bmodule. Then B A B = (A A A) (A A I) (I A A) (I A I) and for i I we have d 0 (i) = i 1 1 i. Now, since d 0 all of A in its kernel, it follows that A = ker d 0, as desired. Alternative proof: Consider the map h := g id : B A B B. Then d 0 (b) = 0 implies that 0 = h d 0 (b) = b g(b), which shows b = g(b) A. 2. Suppose that A C is a faithfully flat extension. Then (B A B) A C = (B A C) C (B A C). Thus, if we tensor (*) with C over A, we obtain 0 C φ B A C d 0 (B A C) C (B A C). Since C is faithfully flat, it suffices to prove that this latter sequence is exact. Thus, we can replace the pair (A, B) with (C, B A C). 3. Finally, consider an arbitrary A B. Now we apply the previous reduction with C = B. Then we get the pair or rings B B A B, b b 1, and we can construct a section by setting g(b b ) = bb. But this puts us in case (1), which completes the proof. In fact, using the same proof method, one can prove the following fairly vast generalization of the previous lemma: Lemma 1.3. Let φ : A B be a faithfuly flat map of rings, and let M be any A module. Then 0 M M A B d 0 M A B 2 dr 2 M A B r 2
3 is an exact sequence of Amodules, where B r = B A A B, r times d r 1 (b) = i 1 i e i e i (b 0 b r 1 ) = b 0 b i 1 1 b i b r 1 Proof. That the composition of any two maps is 0 is straightforward. To check exactness, we reduce as before to the case where there exists a section g : B A. Then consider the morphism k r : B r+2 B r+1 defined as k r (b 0 b r+1 ) = g(b 0 ) b 1 b 2 b r+1. It is easily seen that k r+1 d r+1 + d r k r = id, from which the exactness follows. Proof of theorem 1.1. : First, by setting Y = i I U i we can reduce to the case of a single flat, surjective, locally of finite type map φ : Y X Suppose h : Y Z is a morphism such that h p 1 = h p 2, where p i is the map from Y X Y to Y by projecting onto the i th coordinate. We wish to prove the existence and uniqueness of a morphism g : X Z such that g φ = h. 1. We first prove that there is at most one such map g, so suppose g 1, g 2 are two such maps. Since φ is surjective as a map of topological spaces, it follows that g 1, g 2 agree as maps of topological spaces. Hence, let x X, z = g 1 (x) = g 2 (x) Z and pick y Y such that φ(y) = X. Then we have induced maps O Z,z O X,x O Y,y where the first pair of maps is g # 1, g# 2 and the second map is φ# and the compositions agree and equal h #. Since Y is flat over X and local flat maps are faithfully flat by lecture 3, it follows that φ # is injective, and thus g # 1 = g # 2. Thus g 1, g 2 induce the same maps of stalks at every point, and hence they are the same. 2. By the uniqueness above, it suffices to work locally on X. Thus take x X, and consider y Y such that φ(y) = x and h(y) = z. Now let 3
4 Z be an affine open neighborhood of z, let Y = h 1 (Z ) and consider φ(h 1 (Z )) X. This is open, as φ is an open map, so let X be an affine open neighborhood contained within it. Now as we can work locally on X, replace X, Y, Z by X, Y, Z. Thus we can reduce to the case where X, Z are affine. 3. Write Y = i I V i where V i is affine open. Since X is affine and quasicompact, there is a finite subset K I such that X = k K φ(v k ). Let J be any finite subset of I containing K, and write Y J := j J V j. This is affine, so we can write Y J = Spec B. Likewise, let X = Spec A and Z = Spec C. Now, the sequence can be rewritten Hom(X, Z) Hom(Y J, Z) Hom ( Y J X Y J, Z) hom(c, A) hom(c, B) hom(c, B A B). Since the hom functor is always leftexact, that this sequence is exact follows from lemma 1.2. Thus, there exists a unique map g : X Z inducing h : Y J Z. Since J was an arbitrary finite subset and g is unique, it follows that g φ = h, as desired. Excercise 1.4. Let X, Z be schemes. Prove that F Z (U) := hom(u, Z) is a sheaf on X fl. 2 Descent of Modules and Affine Schemes Let A B be a faithfully flat morphism of rings, and let M be an A module. Then M := M A B is a Bmodule. Moreover, we can define two B A B modules, given by M A B and B A M. Note that while the underlying sets of these two modules are clearly the same, the actions of B A B are very different. Nontheless, in this case we have an isomorphism φ M : B A M = M A B given by φ M (b (m b )) = (m b) b. 4
5 This induces three morphisms φ M,2,3 : B A B A M B A M A B φ M,1,3 : B A B A M M A B A B φ M,1,2 : B A M A B M A B A B by setting φ i,j to be induced by applying φ M on the i, j th coordinates, and one can check that φ M,1,3 = φ M,1,2 φ M,2,3. Remark: One can think of this as a cocycle condition in the following way: Consider M as a sheaf of modules on Spec B. Recall that to give a sheaf on X from sheafs S i on U i where (U i ) i I is an open covering of X, one needs to give isomophisms φ i,j : S i U i U j = Sj U i U j together with the gluing condition φ i,k = φ j,k φ i,j. Now thinking of Spec B as a covering of A, we need to check the same conditions. This looks a little weird as we only have one open set, but becomes nontrivial since our morphisms are now more complicated. Thus, the analogue in this setting is an isomorphism φ 1,2 : p 1M p 2M where p 1, p 2 are the projection maps on Spec B Spec A Spec B, together with the gluing condition φ 1,3 = φ 2,3 φ 1,2. This is precisely what our definition reflects. This discussion the following lemma more intuitive, and justifies our treating faithfully flat morphisms as coverings. Theorem 2.1. Every pair (M, φ) where φ : B A M = M A B satisfies φ 1,3 = φ 1,2 φ 2,3 there is an Amodule M, unique up to isomorphism, with a unique isomorphism M = M A B identifying φ with φ M. Remark. Note that the condition that φ be identified with φ M is crucial for the uniqueness statement, and this will in turn be crucial for the gluing argument once we generalize this to schemes. Proof. Define the Amodule M by setting M = {m M φ(1 m) = m 1}. We claim that the natural map M A B M is an isomorphism, which moreover identifies φ M with φ. To prove this, consider the following diagram: B A M φ M A B 1 α 1 β 1 π 2 1 π 3 B A M A B φ 1,2 M A B A B 5
6 where α(m) = m 1 and β(m) = φ(1 m). The cocycle condition implies that the diagram commutes with either the top or bottom arrows, hence the left downward map φ identifies their kernels. Since B is faithfully flat over A, the upper kernel is B M and by lemma 1.3 the bottom kernel is M. This proves the claim. We are now ready to prove a descent theorem for schemes: Theorem 2.2. Let Y X be faithfully flat. Suppose Z Y is an affine scheme, and φ : Y X Z Z X Y is an isomorphism of Y X Y schemes, such that φ 1,3 and φ 2,3 φ 1,2 induce the same isomorphism Y X Y X Z Z X Y X Y. Then up to isomorphism, there exists a unique pair (Z, ψ) consisting of an affine Xscheme Z X and an isomorphism ψ : Z X Y Y of Y schemes, such that under the identification ψ, the map φ on Y X Z X Y becomes the natural map (y 1, z, y 2 ) (z, y 1, y 2 ). Proof. By the uniqueness claim, we may work locally on X and Y, wlog X = Spec A, Y = Spec B, Z = Spec C. But then the theorem follows immediately from lemma 2.1 if we let M be the Bmodule C. Excercise 2.3. Let φ : Z X be a morphism, and ψ : Y X be a faithfully flat morphism, and φ Y : Z X Y X the base change of φ along ψ. For each of the following properties P, prove that φ is a morphism of type P iff φ Y is: Open Immersion Unramified Finite Type Finite Flat Etale Faithfully Flat Closed Immersion 6
7 3 Locally constant sheaves Let X be a connected scheme. Definition. A sheaf F of sets(resp. abelian groups) on X E is constant if there exits a set(resp. abelian group) S such that F(U) = S π0(u), where π 0 (U) denotes the set of connected components of U. We define the rank if F to be the cardinality of S. A sheaf F is locally constant if there exists a covering (U i X) i I such that the restriction of F to (U i ) E is constant. It is not difficult to see that for any two U i with nonempty fiber product the rank of F on (U i ) E is the same. It thus follows since X is connected that we have a welldefined notion of rank for locally constant sheaves. Lemma 3.1. Let E be et or fl. If Z X is a finite etale cover, then F Z (U) := Hom X (U, Z) defines a locally constant sheaf. Moreover, if Z is another finite etale cover of X, then hom(f Z, F Z ) = hom X (Z, Z ). Proof. That F Z is a sheaf does not use that Z is finite etale, and follows immediately from theorem 1.1 (see Ex. 1.4). Now, it is easy to see that for any Emorphism Y X the restriction of F Z to Y E is F Z X Y. Consider a galois cover Y X such that Y surjects onto Z. By the equivalence of categories between finite etale covers and π 1 (X) sets proven in lecture 6, it follows that Y X Z as a scheme over Y is isomorphic to Y homx(y,z). Thus, F Z restricted to Y E is the locally constant sheaf corresponding to the set S = hom X (Y, Z). For the second part of the lemma, consider φ hom(f Z, F Z ). Then φ Z : F Z (Z) F Z (Z), so φ Z (1 Z ) hom(z, Z ). Conversely, for ψ hom X (Z, Z ) we get a map F Z F Z via composition with ψ. It is straightforward that these two maps are inverses to each other, which proves the claim. Theorem 3.2. The functor Z F Z defines an equivalence of categories between finite etale covers of X and locally constant sheaves of finite rank. Proof. In view of lemma 3.1 it suffices to prove that an arbitrary locally free sheaf F of finite rank is of the form F Z for some Z. Since F is locally free, it follows that there exists a covering (U i X) i I such that the restriction of F to (U i ) E is constant. Let Y = i I U i. Then F is constant on Y E  associated to some finite set S  so there exists a scheme Z = Y S which is finite etale over Y and an isomorphism φ : F Z = F YE. Now, the restriction 7
8 of F to (Y X Y ) E can be thus identified with either F Z X Y or F Y X Z, so we get an isomorphism of Y X Y scheme φ : Y X Z Z X Y. Moreover, restricting to Y X Y X Y shows that φ 1,3 = φ 2,3 φ 1,2. Thus by theorem 2.2 we get a scheme Z X and an identification ψ : Z X Y Z which makes φ into the natural flip the coordinates map. Now theorem 1.1 together with the sheaf condition for F gives us an identification F = F Z. Finally, that Z is itself etale follows from excercise 2.3. As an immediate corollary to theorem 3.2 together with the equivalence of categories between finite etale covers and π 1 (X) sets proven in lecture 6, we have the following Corollary 3.3. Let x be a geometric point of X. The category of locally constant sheaves of sets(resp. abelian groups) is naturally isomorphic to the category of π 1 (X, x)sets(resp. modules). 8
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