The Riemann-Roch Theorem

Transcription

1 The Riemann-Roch Theorem Paul Baum Penn State Texas A&M University College Station, Texas, USA April 4, 2014

2 Minicourse of five lectures: 1. Dirac operator 2. Atiyah-Singer revisited 3. What is K-homology? 4. Beyond ellipticity 5. The Riemann-Roch theorem The minicourse is based on joint work with Ron Douglas and is dedicated to Ron Douglas.

3 THE RIEMANN-ROCH THEOREM Topics in this talk : 1. Classical Riemann-Roch 2. Hirzebruch-Riemann-Roch (HRR) 3. Grothendieck-Riemann-Roch (GRR) 4. RR for possibly singular varieties (Baum-Fulton-MacPherson)

4 CLASSSICAL RIEMANN - ROCH M compact connected Riemann surface genus of M = # of holes = 1 2 [rankh 1(M; Z)]

5 D a divisor of M D consists of a finite set of points of M p 1, p 2,..., p l and an integer assigned to each point n 1, n 2,..., n l Equivalently D is a function D : M Z with finite support Support(D) = {p M D(p) 0} Support(D) is a finite subset of M

6 D a divisor on M deg(d) := p M D(p) Remark D 1, D 2 two divisors D 1 D 2 iff p M, D 1 (p) D 2 (p) Remark D a divisor, D is ( D)(p) = D(p)

7 Example Let f : M C { } be a meromorphic function. Define a divisor δ(f) by: 0 if p is neither a zero nor a pole of f δ(f)(p) = order of the zero if f(p) = 0 (order of the pole) if p is a pole of f

8 Example Let ω be a meromorphic 1-form on M. Locally ω is f(z)dz where f is a (locally defined) meromorphic function. Define a divisor δ(ω) by: 0 if p is neither a zero nor a pole of ω δ(ω)(p) = order of the zero if ω(p) = 0 (order of the pole) if p is a pole of ω

9 D a divisor on M { } meromorphic functions H 0 (M, D) := δ(f) D f : M C { } { } meromorphic 1-forms H 1 (M, D) := ω on M δ(ω) D Lemma H 0 (M, D) and H 1 (M, D) are finite dimensional C vector spaces dim C H 0 (M, D) < dim C H 1 (M, D) <

10 Theorem (RR) Let M be a compact connected Riemann surface and let D be a divisor on M. Then: dim C H 0 (M, D) dim C H 1 (M, D) = d g + 1 d = degree (D) g = genus (M)

11 HIRZEBRUCH-RIEMANN-ROCH M non-singular projective algebraic variety / C E an algebraic vector bundle on M E = sheaf of germs of algebraic sections of E H j (M, E) := j-th cohomology of M using E, j = 0, 1, 2, 3,...

12 LEMMA For all j = 0, 1, 2,... dim C H j (M, E) <. For all j > dim C (M), H j (M, E) = 0. χ(m, E) := n = dim C (M) n ( 1) j dim C H j (M, E) j=0 THEOREM[HRR] Let M be a non-singular projective algebraic variety / C and let E be an algebraic vector bundle on M. Then χ(m, E) = (ch(e) T d(m))[m]

13 Hirzebruch-Riemann-Roch Theorem (HRR) Let M be a non-singular projective algebraic variety / C and let E be an algebraic vector bundle on M. Then χ(m, E) = (ch(e) T d(m))[m]

14 EXAMPLE. Let M be a compact complex-analytic manifold. Set Ω p,q = C (M, Λ p,q T M) Ω p,q is the C vector space of all C differential forms of type (p, q) Dolbeault complex 0 Ω 0,0 Ω 0,1 Ω 0,2 Ω 0,n 0 The Dirac operator (of the underlying Spin c manifold) is the assembled Dolbeault complex + : j Ω 0, 2j j 0, 2j+1 Ω The index of this operator is the arithmetic genus of M i.e. is the Euler number of the Dolbeault complex.

15 K-theory and K-homology in algebraic geometry Let X be a (possibly singular) projective algebraic variety / C. Grothendieck defined two abelian groups: Kalg 0 (X) = Grothendieck group of algebraic vector bundles on X. K alg 0 (X) = Grothendieck group of coherent algebraic sheaves on X. Kalg 0 (X) = the algebraic geometry K-theory of X contravariant. K alg 0 (X) = the algebraic geometry K-homology of X covariant.

16 K-theory in algebraic geometry Vect alg X = set of isomorphism classes of algebraic vector bundles on X. A(Vect alg X) = free abelian group with one generator for each element [E] Vect alg X. For each short exact sequence ξ 0 E E E 0 of algebraic vector bundles on X, let r(ξ) A(Vect alg X) be r(ξ) := [E ] + [E ] [E]

17 K-theory in algebraic geometry R A(Vect alg (X)) is the subgroup of A(Vect alg X) generated by all r(ξ) A(Vect alg X). DEFINITION. K 0 alg (X) := A(Vect algx)/r Let X, Y be (possibly singular) projective algebraic varieties /C. Let f : X Y be a morphism of algebraic varieties. Then have the map of abelian groups f : K 0 alg (X) K0 alg (Y ) [f E] [E] Vector bundles pull back. f E is the pull-back via f of E.

18 K-homology in algebraic geometry S alg X = set of isomorphism classes of coherent algebraic sheaves on X. A(S alg X) = free abelian group with one generator for each element [E] S alg X. For each short exact sequence ξ 0 E E E 0 of coherent algebraic sheaves on X, let r(ξ) A(S alg X) be r(ξ) := [E ] + [E ] [E]

19 K-homology in algebraic geometry R A(S alg (X)) is the subgroup of A(S alg X) generated by all r(ξ) A(S alg X). DEFINITION. K alg 0 (X) := A(S alg X)/R Let X, Y be (possibly singular) projective algebraic varieties /C. Let f : X Y be a morphism of algebraic varieties. Then have the map of abelian groups f : K alg 0 (X) K alg 0 (Y ) [E] Σ j ( 1) j [(R j f)e]

20 f : X Y morphism of algebraic varieties E coherent algebraic sheaf on X For j 0, define a presheaf (W j f)e on Y by U H j (f 1 U; E f 1 U) U an open subset of Y Then (R j f)e := the sheafification of (W j f)e

21 f : X Y morphism of algebraic varieties f : K alg 0 (X) K alg 0 (Y ) [E] Σ j ( 1) j [(R j f)e]

22 SPECIAL CASE of f : K alg 0 (X) K alg 0 (Y ) Y is a point. Y = ɛ: X is the map of X to a point. Kalg 0 ( ) = Kalg 0 ( ) = Z ɛ : K alg 0 (X) K alg 0 ( ) = Z ɛ (E) = χ(x; E) = Σ j ( 1) j dim C H j (X; E)

23 X non-singular = K 0 alg (X) = K alg 0 (X) Let X be non-singular. Let E be an algebraic vector bundle on X. E denotes the sheaf of germs of algebraic sections of E. Then E E is an isomorphism of abelian groups Kalg 0 (X) Kalg 0 (X) This is Poincaré duality within the context of algebraic geometry K-theory&K-homology.

24 X non-singular = K 0 alg (X) = K alg 0 (X) Let X be non-singular. The inverse map is defined as follows. K alg 0 (X) K 0 alg (X) Let F be a coherent algebraic sheaf on X. Since X is non-singular, F has a finite resolution by algebraic vector bundles.

25 X non-singular = K 0 alg (X) = K alg 0 (X) F has a finite resolution by algebraic vector bundles. i.e. algebraic vector bundles on X E r, E r 1,..., E 0 and an exact sequence of coherent algebraic sheaves 0 E r E r 1... E 0 F 0 Then K alg 0 (X) Kalg 0 (X) is F Σ j ( 1) j E j

26 Grothendieck-Riemann-Roch Theorem (GRR) Let X, Y be non-singular projective algebraic varieties /C, and let f : X Y be a morphism of algebraic varieties. Then there is commutativity in the diagram : K 0 alg (X) K0 alg (Y ) ch( ) T d(x) ch( ) T d(y ) H (X; Q) H (Y ; Q)

27 WARNING!!! The horizontal arrows in the GRR commutative diagram K 0 alg (X) K0 alg (Y ) ch( ) T d(x) ch( ) T d(y ) are wrong-way (i.e. Gysin) maps. H (X; Q) H (Y ; Q) K 0 alg (X) = K alg 0 (X) f K alg 0 (Y ) = K 0 alg (Y ) H (X; Q) = H (X; Q) f H (Y ; Q) = H (Y ; Q) Poincaré duality Poincaré duality

28 K-homology is the dual theory to K-theory. How can K-homology be taken from algebraic geometry to topology? There are three ways in which this has been done: Homotopy Theory K-homology is the homology theory determined by the Bott spectrum. K-Cycles K-homology is the group of K-cycles. C* algebras K-homology is the Atiyah-BDF-Kasparov group KK (A, C).

29 Riemann-Roch for possibly singular complex projective algebraic varieties Let X be a (possibly singular) projective algebraic variety / C Then (Baum-Fulton-MacPherson) there are functorial maps α X : K 0 alg (X) K0 top(x) K-theory contravariant natural transformation of contravariant functors β X : K alg 0 (X) K top 0 (X) K-homology covariant natural transformation of covariant functors Everything is natural. No wrong-way (i.e. Gysin) maps are used.

30 α X : K 0 alg (X) K0 top(x) is the forgetful map which sends an algebraic vector bundle E to the underlying topological vector bundle of E. α X (E) := E topological

31 Let X, Y be projective algebraic varieties /C, and let f : X Y be a morphism of algebraic varieties. Then there is commutativity in the diagram : K 0 alg (X) K0 alg (Y ) α X α Y K 0 top(x) K 0 top(y ) i.e. natural transformation of contravariant functors

32 Let X, Y be projective algebraic varieties /C, and let f : X Y be a morphism of algebraic varieties. Then there is commutativity in the diagram : K 0 alg (X) K0 alg (Y ) α X α Y K 0 top(x) K 0 top(y ) ch ch H (X; Q) H (Y ; Q)

33 Let X, Y be projective algebraic varieties /C, and let f : X Y be a morphism of algebraic varieties. Then there is commutativity in the diagram : K alg 0 (X) K alg 0 (Y ) β X β Y K top 0 (X) Ktop 0 (Y ) i.e. natural transformation of covariant functors Notation. K top is K-cycle K-homology.

34 Let X, Y be projective algebraic varieties /C, and let f : X Y be a morphism of algebraic varieties. Then there is commutativity in the diagram : K 0 alg (X) K0 alg (Y ) α X α Y K 0 top(x) K 0 top(y ) ch ch H (X; Q) H (Y ; Q)

35 Let X, Y be projective algebraic varieties /C, and let f : X Y be a morphism of algebraic varieties. Then there is commutativity in the diagram : K alg 0 (X) K alg 0 (Y ) β X β Y K top 0 (X) Ktop 0 (Y ) ch ch H (X; Q) H (Y ; Q)

36 Definition of β X : K alg 0 (X) K top 0 (X) Let F be a coherent algebraic sheaf on X. Choose an embedding of projective algebraic varieties where W is non-singular. ι: X W ι F is the push forward (i.e. extend by zero) of F. ι F is a coherent algebraic sheaf on W.

37 ι F is a coherent algebraic sheaf on W. Since W is non-singular, ι F has a finite resolution by algebraic vector bundles. Consider 0 E r E r 1... E 0 ι F 0 0 E r E r 1... E 0 0 These are algebraic vector bundles on W and maps of algebraic vector bundles such that for each p W ι(x) the sequence of finite dimensional C vector spaces is exact. 0 (E r ) p (E r 1 ) p... (E 0 ) p 0

38 Choose Hermitian structures for E r, E r 1,..., E 0 Then for each vector bundle map there is the adjoint map σ : E j E j 1 σ : E j E j 1 σ σ : j E 2j j E 2j+1 is a map of topological vector bundles which is an isomorphism on W ι(x).

39 Let Ω be an open set in W with smooth boundary Ω such that Ω = Ω Ω is a compact manifold with boundary which retracts onto ι(x). Ω ι(x). Set M = Ω Ω Ω M is a closed Spin c manifold which maps to X by: ϕ: M = Ω Ω Ω Ω ι(x) = X

40 On M = Ω Ω Ω let E be the topological vector bundle E 2j (σ σ ) E 2j+1 E = j Then β X : K alg 0 (X) K top 0 (X) is : F (M, E, ϕ) j

41 Module structure Kalg 0 (X) is a ring and Kalg 0 (X) is a module over this ring. α X : K 0 alg (X) K0 top(x) is a homomorphism of rings. β X : K alg 0 (X) K top 0 (X) respects the module structures.

42 Todd class Set td(x) = ch (β X (O X )) td(x) H (X; Q) If X is non-singular, then td(x) = Todd(X) [X]. With X possibly singular and E an algebraic vector bundle on X χ(x, E) = ɛ (ch(e) td(x)) ɛ: X is the map of X to a point. ɛ : H (X; Q) H ( ; Q) = Q