3. Lecture 3. Y Z[1/p]Hom (Sch/k) (Y, X).


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1 3. Lecture Freely generate qfhsheaves. We recall that if F is a homotopy invariant presheaf with transfers in the sense of the last lecture, then we have a well defined pairing F(X) H 0 (X/S) F(S) given by associating to any irreducible i : Y X finite, surjective over S the mapping T r Y/S i : F(X) F(Y ) F(S). Moreover, recall that any qfhsheaf admits transfers. Consider the following presheaf, where Z[1/p] denotes localization of the residue characteristic of k (so that if k is of characteristic 0, then Z[1/p] should be read as Z): (Sch/k) op Ab, Y Z[1/p]Hom (Sch/k) (Y, X). We denote by F X the associated qfhsheaf. The following result of SuslinVoevodsky identifies this sheaf when applied to normal varieties and relates this to the Suslin complex Sus (X) of X. One can interpret this theorem as saying that cycles in X Y each component of which is finite and surjective over a normal variety Y are locally in the qfhtopology a sum of graphs of morphisms from Y to X. Theorem 3.1. If Y (Sch/k) is normal, then F X (Y ) = C 0 (X Y/Y ) Z[1/p]. In particular, F X ( ) = Sus (X) Z[1/p]. Sheafifying in the qfhtopology gives us transfers. The following proposition enables us to obtain presheaves which are homotopy invariant. Proposition 3.2. Assume that F is a presheaf with transfers. Let (F ) denote the complex of sheaves given in degree q as the qfh sheaf associated to the presheaf Y F(Y q ). For any q 0, consider the presheaf H q ((F ) ) : (Sch/k) op Ab defined as the homology sheaves of the complex (F ) (or, equivalently, the sheaf associated to the presheaf Y H q (F(Y ))). Then H q ((F ) ) is a homotopy invariant presheaf with transfers. Proof. One first shows that evaluation at 0, 1 1 determine chain homotopy maps F(Y 1 ) F(Y ). This enables one to show that upon taking qfhsheaves that H q ((F ) ) is homotopy invariant. Moreover, the naturality properties of transfers on F imply that T r Y/S gives us a transfer map on complexes F(X ) F(S ), so that taking associated qfhsheaves gives us transfers on H q ((F ) ). 1
2 Proof of SuslinVoevodsky theorem. We shall sketch a proof of the following theorem. Theorem 3.3. (Suslin Voevodsky [2]) Let F X denote the qfhsheaf associated to the prsheaf (Sch/k) op Ab, Y Z[1/p]Hom (Sch/k) (Y, X). Then for any positive integer n invertible in k, the natural maps of complexes of qf hsheaves F X ( ) (F X ) F X induce isomorphisms of Extgroups ext (3.3.1) H (Sus (X), Z/n) Ext qfh((f X ), Z/n) Ext qfh(f X, Z/n). Moreover, Ext qfh (F X, Z/n) Ext (X et, Z/n). Proof. The two isomorphisms of (3.3.1) are proved considering the two hypercohomology spectral sequences for Ext qfh ((F X ), Z/n). The first isomorphism follows by comparing the homology at each level of the map of complexes of presheaves with transfers F X ( ) Z/n (F X ) Z/n, where the left hand complex is viewed as a complex of constant presheaves. Since the homology presheaves of these complexes are presheaves with transfer which are annihilated by multiplication by n, we can apply the SuslinVoevodsky theorem to conclude that the induced map on homology sheaves is an isomorphism. The second isomorphism does not use the fact that F X is a presheaf with transfers, but is a general fact that F X (F X ( q )) induces an isomorphism in Ext qfh (, Z/n). The right had side is almost by definition H (X qfh, Z/n). The comparison with the etale cohomology of X is achieved by realizing explicitly sufficiently fine qfh coverings together with resolution of singularities (in characteristic p > 0, one uses de Jong s modifications rather than resolutions which are not known to exist Discussion of Chow groups. Recall that X is said to be integral if O X (U) is an integral domain for all open subsets U X. The field of fractions K of such an integral variety is the field of fractions of O X (U) for any affine open subset U. If O X (U) is integrally closed in K for every affine open subset U, then the stalk O X,x at any (schemetheoretic point) x X of codimension 1 is a discrete valuation ring. Definition 3.4. Let X is an integral variety regular in codimension 1 and let K be its field of fractions. For any 0 f K, we define the principal divisor (f) associated to f to be the following formal sum of codimension 1, irreducible subvarieties (f) = v x (f)x. x X (1) Here, X (1) X consists of the schemetheoretic points of codimension 1, v x : K Z is the discrete valuation at x X (1), and x X is the codimension 1 irreducible subvariety of X given as the closure of x. A formal sum D = n x x, n x Z x X (1)
3 3 with all but finitely many n x equal to 0 is said to be a locally principal divisor provided that for every x X (1) there exists some Zariski open neighborhood x U x X and some f x K such that D Ux = (f x ) Ux. Definition 3.5. Let X be a quasiprojective algebraic variety. An algebraic rcycle on X if a formal sum n Y [Y ], Y irreducible of dimension r, n Y Z Y with all but finitely many n Y equal to 0. Equivalently, an algebraic rcycle is a finite integer combination of points of X of dimension r. If Y X is a subvariety each of whose irreducible components Y 1,..., Y m is rdimensional, then the algebraic rcycle m Z = [Y i ] i=1 is called the cycle associated to Y. The group of (algebraic) rcycles on X will be denoted Z r (X). Two rcycles Z, Z on a quasiprojective variety X if their difference lies in the subgroup Z r,rat (X) Z r (X) generated by cycles of the form W X {p} W X {q}, where U P 1 is a Zariski open set containing points p, q U and W X U is a cycle each of whose irreducible components maps surjectively onto U. The Chow group CH r (X) = Z r (X)/Z r,rat (X) is the group of rcycles modulo rational equivalence. Theorem 3.6. (cf. [1]) Assume that X is an integral variety regular in codimension 1. Let D(X) denote the group of locally principal divisors on X modulo principal divisors. Then there is a natural isomoprhism P ic(x) D(X). If L P ic(x) has a nonzero global section s L(X) = Γ(X, L), then this isomorphism sends L to x X (1) v x(s)x. Moreover, if O X,x is a unique factorization domain for every x X, then D(X) CH 1 (X), the Chow group of codimension 1 cycles on X modulo rational equivalence. Remark 3.7. Not only is this an example of relating bundles to cycles, but it is also an example of duality. Namely, P ic(x) is contravariant, whereas CH r ( ) is covariant for proper maps. This suggests that for P ic(x) to be isomorphic to CH 1 (X), some smoothness condition on X is required. Observe that in the above definition we can replace the role of r + 1cycles on X P 1 and their geometric fibres over 0, by r + 1cycles on X U for any nonempty Zaristik open U X and geometric fibres over any two krational points p, q U. Remark 3.8. Given some r +1 dimensional irreducible subvariety V X together with some f k(v ), we may define (f) = S ord S(f)[S] where S runs through the codimension 1 irreducible subvarieties of V. Here, ord S ( ) is the valuation
4 4 centered on S if V is regular at the codimension 1 point corresponding to S; more generally, ord S (f) is defined to be the length of the O V,S module O V,S /(f). We readily check that (f) is rationally equivalent to 0: namely, we associate to (V, f) the closure W = Γ f X P 1 of the graph of the rational map V P 1 determined by f. Then (f) = W X {0} W X { }. Conversely, given an r+1dimensional irreducible subvariety W on X P 1 which maps onto P 1, the composition W X P 1 pr2 P 1 determines f frac(w ) such that (f) = W X {0} W X { }. Thus, the definition of rational equivalence on rcycles of X can be given in terms of the equivalence relation generated by {(f), f frac(w ); W irreducible of dimension r + 1} In particular, we conclude that the subgroup of principal divisors inside the group of all locally principal divisors consists precisely of those locally principal divisors which are rationally equivalent to 0. Example 3.9. For essentially formal reasons, P ic(x) H 1 (X, OX ). If k is the complex field, we can use the exponential sequence of sheaves in the analytic topology exp 0 Z O X OX 0 to conclude that the kernel of H 1 (X an, OX ) H2 (X an, Z) is the complex vector space H 1 (X an, O X ) modulo the discrete subgroup H 1 (X an, Z). For example, if X = C is a smooth, projective curve of genus g, then CH 1 (C) fits in a short exact sequence 0 C g /Z 2g = H 1 (C, O C)/H 1 (C an, Z) CH 1 (C) Z = H 2 (C an, Z) 0. Example Let X = A N. Then any N 1cycle (i.e., Weil divisor) Z CH N 1 (A N ) is principal, so that CH N 1 (A N ) = 0. More generally, consider the map µ : A N A 1 P N A 1 which sends (x 1,..., x n ), t to t x 1,..., t x n, 1, t. Consider an ireducible subvariety Z A N of dimension r > N not containing the origin and Z P N be its closure. Let W = µ 1 (Z A 1 ). Then W [0] = whereas W [1] = Z. Thus, CH r (A N ) = 0 for any r < N. Example Arguing in a similar geometric fashion, we see that the inclusion of a linear plane P N 1 P N induces an isomorphism CH r (P N 1 ) = CH r (P N ) provided that r < N and thus we conclude by induction that CH r (P N ) = Z if r N. Namely, consider µ : P N A 1 P N A 1 sending x 0,..., x N, t to x ),..., x N 1, t x N, t and set W = µ 1 (Z A 1 ) for any Z not containing 0,..., 0, 1. Then W [0] = pr N (Z), W [1] = Z. Example Let C be a smooth curve. Then P ic(c) CH 0 (X). Definition If f : X Y is a proper map of quasiprojective varieties, then the proper pushforward of cycles determines a well defined homomorphism f : CH r (X) CH r (Y ), r 0. Namely, if Z X is an irreducible subvariety of X of dimension r, then [Z] is sent to d [f(z)] CH r (Y ) where [k(z) : k(f(z))] = d if dim Z = dim f(z) and is sent to 0 otherwise.
5 5 If g : W X is a flat map of quasiprojective varieties of relative dimension e, then the flat pullback of cycles induces a well defined homomorphism g : CH r (X) CH r+e (W ), r 0. Namely, if Z X is an irreducible subvariety of X of dimension r, then [Z] is sent to the cycle on W associated to Z X W W. Proposition Let Y be a closed subvariety of X and let U = X\Y. i : Y X, j : U X be the inclusions. Then the sequence Let CH r (Y ) i CH r (X) j CH r (U) 0 is exact for any r 0. Proof. If V U is an irreducible subvariety of U of dimension r, then the closure of V in X, V X, is an irreducible subvariety of X of dimension r with the property that j ([V ]) = [V ]. Thus, we have an exact sequence Z r (Y ) i Z r (X) j Z r (U) 0. If Z = i n i[y i ] is a cycle on X with j (Z) = 0 CH r (U), then j Z = W,f (f) where each W U is an irreducible subvarieties of U of dimension r + 1 and f k(w ). Thus, Z = i n i[y i ] W,f (f) is an rcycle on Y with the property that i (Z ) is rationally equivalent to Z. Exactness of the asserted sequence of Chow groups is now clear. Corollary Let H P N be a hypersurface of degree d. Then CH N 1 (P N \H) = Z/dZ. Example Mumford shows that if S is a projective smooth surface with a nonzerol global algebraic 2form (i.e., H 0 (S, Λ 2 (Ω S )) 0), then CH 0 (S) is not finite dimensional (i.e., must be very large). Bloch s Conjecture predicts that if S is a projective, smooth surface with geometric genus equal to 0 (i.e., H 0 (S, Λ 2 (Ω S )) = 0), then the natural map from CH 0 (S) to the (finite dimensional) Albanese variety is injective Intersection product. Theorem Let X be a smooth quasiprojective variety of dimension d. Then there exists a pairing CH r (X) CH s (X) CH d r s, d r + s, with the property that if Z = [Y ], Z = [W ] are irreducible cycles of dimension r, s respectively and if Y W has dimension d r s, then Z Z is a cycle which is a sum with positive coefficients indexed by the irreducible subvarieties of Y W of dimension d r s. For notational purposes, we shall often write CH s (X) for CH d s (X). With this indexing convention, the intersection pairing has the form CH s (X) CH t (X) CH s+t (X).
6 6 Proof. Classically, this was proved by showing the following geometric fact: given a codimension r cycle Z and a codimension s cycle W = j m jr j with r + s d, then there is another codimension r cycle Z = i n iy i rationally equivalent to Z (i.e., determining the same element in CH r (X)) such that Z meets W properly ; in other words, every component C i,j,k of each Y i R j has codimension r + s. One then defines Z W = i,j,k n i m j int(y i R j, C i,j,k )C i,j,k where int(y i R j, C i,j,k ) is a positive integer determined using local commutative algebra, the intersection multiplicity. Furthermore, one shows that if one chooses a Z rationally equivalent to both Z, Z and also intersecting W properly, then Z W is rationally equivalent to Z W. To be continued next lecture... References Hart [1] R. Hartshorne, Algebraic Geometry, Springer SV [2] A. Suslin and V. Voevodsky, Singular homology of abstract algebraic varieties. Inventiones Math. 123 (1996), Department of Mathematics, Northwestern University, Evanston, IL address:
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