# Cohomology and Base Change

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1 Cohomology and Base Change Let A and B be abelian categories and T : A B and additive functor. We say T is half-exact if whenever 0 M M M 0 is an exact sequence of A-modules, the sequence T (M ) T (M) T (M ) is exact. Lemma 1 Let η: T T be a morphism of half exact functors, and for each M, let T (M) be the cokernel of η M. Then T is half-exact if T is right exact. Proof: If 0 M M M 0 is an exact sequece, we get a commutative diagram: T (M ) T (M) T (M ) 0 T (M ) T (M) T (M ) T (M ) T (M) T (M ) The rows and columns are exact and the bottom vertical arrows are surjective. A diagram chase shows that the bottom row is exact. The following useful lemma is not especially well known. Theorem 2 (Nakayama s lemma for half-exact functors) Let A B be a local homomorism of noetherian local rings and T a half-exact functor A-linear functor from the category of finitely generated A-modules to the category of finitely generated B-modules. Let k be the residue field of A. Then if T (k) = 0, in fact T (M) = 0 for all M. Proof: Our hypothesis is that T (k) = 0, and we want to conclude that T (M) = 0 for every finitely generated A-module M. Consider the family F of submodules M of M such that T (M/M ) 0. Our claim is that the 0 submodule does not belong to F, and so of course it will suffice to prove that F is empty. Assuming otherwise, we see from the fact that M is noetherian that F has a maximal element M. Let M := M/M. Then T (M ) 0, but T (M ) = 0 for every nontrivial quotient M of M. We shall see that this leads to a contradiction. To simply the notation, we replace M by M. Thus it suffices to prove that T (M) = 0 under the assumption that T (M ) = 0 for every proper quotient of M. Let I be the annihilator of M and let m denote the maximal ideal of A. If I = m, then M is isomorphic to a direct sum of a (finite number of) copies of k. By assumption, T (k) = 0, hence T (M) = 0, a contradiction. So there exists 1

2 an element a of m such that a I. Let M be the kernel of multiplication of a on M, so that there are exact sequences: and hence also exact sequences: 0 M M am 0 0 am M M/aM 0, T (M ) T (M) T (am) T (am) T (M) T (M/aM). Then am 0, since a does not belong to the annihilator of M, and hence M/aM is a proper quotient of M, and hence T (M/aM) = 0. Case 1: If M 0, then the first sequence above shows that am is also a proper quotient of M, and hence also T (am) = 0, and then the last sequence implies that T (M) = 0. Case 2: If M = 0, we find exact sequences 0 M a M M/aM 0 T (M) a T (M) T (M/aM) However we still know that T (M/aM) = 0, hence multiplication by a on T (M) is surjective. But T (M) is a finitely generated B-module and multiplication by a on this module is the same as multiplication by θ(a). Since θ(a) belongs to the maximal ideal of B, Nakayama s lemma implies that T (M) = 0, as required. Here are two important appications to flatness. Theorem 3 (local criterion for flatness) Let θ: A B be a local homomorphism of noetherian local rings and let N be a finitely generated B-module. Then N is flat as an A-module if and only if T or A 1 (N, A/m) = 0. Proof: To prove that N is flat as an A-module, it suffices to prove that T or1 A (N, M) = 0 for all finitely generated A-modules M. The functor M T or1 A (N, M) is half-exact, A-linear, and takes finitely generated A-modules to finitely generated B-modules. Thus the result follows from Nakayama s lemma for the half exact functor T or1 A (N, ). Theorem 4 (Criterion of Flatness along the Fiber) Let R A and A B be local homomorphisms of noetherian local rings, let k be the residue field of R, and let N be a finitely generated B-module. If N is flat over R and N R k is flat over A R k, then N is flat over A. Proof: We will need the following: Lemma 5 Let A be a ring, let I be an ideal of A, and let M be an A-module. Suppose that M/IM is flat as an A/I-module and also that T or A 1 (A/I, M) = 0. Then T or A 1 (A/J, M) = 0 for every ideal J containing I. 2

3 Proof: Let 0 K F M 0 be an exact sequence of A-modules, with F free. Since Tor A 1 (A/I, M) = 0, the sequence ( ) 0 K/IK F/IF M/IM 0 is an exact sequence of A/I-modules. Since M/IM is flat over A/I, the sequence remains exact if we tensor over A/I with A/J, so the sequence 0 K/JK F/JF M/JM 0 is still exact. However, F is free over A, and we also have an exact sequence 0 T or A 1 (M, A/J) K/JK F/JF M/JM 0 Hence Tor A 1 (A/J, M) = 0. Now to prove the theorem, let m R be the maximal ideal of R and let I := m R A. Then we have a surjective map m R R A I, and hence also a surjective map: m R R A A N I A N. Since m R R A A N = m R R N, we find maps m R R N f I A N g N We have just seen that f is surjective. On the other hand, the kernel of g f is Tor 1 R(k, N) = 0, so g f is injective. Then it follows that f is also injective, hence an isomorphism, and hence that g is injective. The kernel of g is Tor A 1 (A/I, N), so this also vanishes. Furthermore, N/IN = N A (A/I) = N A A R k = N R k, and since N is flat over R, N R k is flat over A R k = A/I. By the lemma, it follows that Tor A 1 (A/J, N) = 0 for every ideal J containing I and in particular for J equal to the maximal ideal of A. By the local criterion for flatness, this implies that N is flat over A. Proposition 6 Let A be a ring, and let T be an A-linear homomorphism from the category of A-modules to itself. Then there is a natural transformation η: T (A) T. This functor is an isomorphism if and only if T is right exact and commutes with direct limits. Proof: An element x of M defines a homomorphism θ x : A M and hence a homomorphism T (θ x ): T (A) T (M). Define T (A) M T (M) by (t, x) θ x (t). This map is bilinear and hence defines a natural homomorphism η M : T (A) M T (M). Since T is additive, it commutes with finite direct sums, and hence η M is an isomorphism if M is free and finitely generated. Assume that T is right exact and commuts with direct limits. For any M, there is an exact sequence: 0 K E M 0, 3

4 with E free and finitely generated, and hence a commutative diagram: T (A) K T (A) E T (A) M 0 T (K) T (E) T (M) 0 The middle vertical arrow is an isomorphism, and it follows that the right vertical arrow is surjective. Since A is noetherian, K is finitely generated, and we can conclude that the left vertical arrow is surjective. Then the right vertical arrow is an isomorphism. This proves that η M is an isomorphism if M is finitely generated, and the same is true for all M since both sides commute with direct limits. The converse is obvious, since tensor products are right exact and commute with direct limits. Corollary 7 Let A B be a local homomorphism of noetherian local rings and let T be a half exact and linear functor from the category of noetherian A-modules to itself. Assume that T takes finitely generated modules to finitely generated modules and commutes with direct limits. 1. If T (k) = 0, then T (M) = 0 for all M. 2. If T (A) T (k) is surjective, then T is right exact and the natural transformation T (A) T is an isomorphism. Proof: Statement (1) follows from Nakayama s lemma for half-exact functors. To prove (2), let T (M) := T (A) A M and let T (M) be the cokernel of the map η M T (M) T (M) defined above. The functor T takes finitely generated modules to finitely generated modules and is is half exact by Lemma 1. By Nakayama s lemma for half-exact functors, T = 0. Then η M is surjective for all M. It follows easily that T is right exact, and hence that η M is an isomorphism by Proposition 6. Note that for statement (2), we just need to assume that T (A) is finitely generated. Theorem 8 (cohomology and base change) Let X/S be a proper scheme, where S = Spec A and A is a noetherian local ring, and let E be a coherent sheaf on X which is flat over S. If M is an A-module, let E M denote the quasi-coherent sheaf on X obtained by tensoring the pullback of M to X with E. In particular, if M is the residue field of A, E M identifies with restriction of E to the closed fiber X k over k. Then the following statement hold. 1. If H q (X k, E k ) = 0, then H q (X, E M) = 0 for all M. 2. If H q (X, E) A k H q (X k, E k ) is surjective, then it is an isomorphism, and in fact H q (X, E) A M H q (X, E M) is an isomorphism for all M. 4

5 3. Suppose that H q (X, E) A k H q (X k, E k ) is surjective. Then H q (X, E) is flat if and only if H q 1 (X, E) k H q 1 (X k, E k ) is surjective. Proof: For each q, let T q be the functor taking an A-module M to H q (X, E M). Since E is flat, an exact sequence 0 M M M 0 yields an exact sequence 0 E M E M E M 0, and so the functors T fit into a cohomological δ-functor. In particiular, each T q is half-exact. Furthermore it takes finitely generated modules to finitely generated modules because X/A is proper, and it commutes with direct limits by a Cech calculation, or using the fact that X is noetherian. Thus statements (1) and (2) follow from the corresponding statements of Corollary 7. To prove (3), note that since T forms a cohomological δ-functor, T q is left exact if and only if T q 1 is right exact. If H q (X, E) k H q (X k, E k ) is surjective, then T q (M) = H q (E M) = H q (E) A M for all M. Thus H q (E) is flat if and only if T q is left exact, which is the case if and only if T q 1 is right exact, equivalently, if and only if H q 1 (X, E) k H q 1 (X k, E k ) is surjective. 5

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