# CHARACTERISTIC CLASSES

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1 1 CHARACTERISTIC CLASSES Andrew Ranicki Index theory seminar 14th February, The Index Theorem identifies Introduction analytic index = topological index for a differential operator on a compact manifold M which has such indices. The right hand side is an expression in terms of the characteristic classes in H (M) of vector bundles over M, such as the tangent bundle TM. We shall follow Roe s book to outline the Chern-Weil method of constructing characteristic classes. The method uses differential geometry via the curvature of a connection on a vector bundle, and algebra via polynomial invariants of matrices.

2 The algebraic Chern-Weil There is a very nice algebraic topology construction of the Chern-Weil method in Bott s 1973 paper On the Chern-Weil homomorphism and the continuous cohomology of Lie groups in the form of a morphism from the invariant forms on the Lie algebra g of a Lie group G to the cohomology of the classifying space BG φ : Inv G (Sg ) H (BG). For a matrix Lie group G the Lie algebra g consists of the matrices X such that exp(tx ) G for all t R. Let G = U(m), the Lie group of unitary m m matrices. Then BU(m) is the classifying space for m-dimensional complex vector bundles. The universal Chern classes are the images c k = φ(c k (X )) H 2k (BU(m)) of the invariant polynomials c k (X ) described in Lemma 2.19 below. However, it is the original differential geometry aspect of the 1950 s Chern-Weil method which is particularly relevant to index theory 3 Characteristic classes 4 Characteristic classes are cohomology classes used to distinguish vector bundles. Isomorphic bundles have the same characteristic classes. The bundles may be real or complex. The cohomology may be taken with various coefficients. A characteristic class γ of a vector bundle V over a space M is a collection of cohomology classes γ (V ) = {γ k (V ) H d k (M) k 0} for some sequence 0 d 0 < d 1 <.... Require γ (V ) to be defined for every V, to satisfy: (naturality) f γ k (V ) = γ k (V ) H d k (M ) for any bundle map (f, b) : (M, V ) (M, V ), (dimension) if V is m-dimensional γ k (V ) = 0 H d k (M) for d k > m, The characteristic classes of a manifold M are γ k (M) = γ k (TM) H d k (M).

3 The Euler class For an m-dimensional real vector bundle V over a space M the sphere and disk bundles S(V ) D(V ) V define a a fibration An oriented V has a Thom class (D m, S m 1 ) (D(V ), S(V )) M. U H m (D(V ), S(V )) = H 0 (M) which restricts to 1 H m (D m, S m 1 ) = Z on each fibre. The Euler characteristic class of V is the image e(v ) = [U] H m (D(V ); Z) = H m (M; Z). e(v ) is the primary obstruction to the existence of a non-zero section. Complete obstruction if M is an m-dimensional complex. The homology groups H (S(V )) of the sphere bundle S m 1 S(V ) M fit into the Wang exact sequence H r (S(V )) H r (M) e(v ) H r m (M) H r 1 (S(V )) The prototypical index theorem 6 The Euler characteristic of a finite-dimensional complex M is χ(m) = ( ) r dim Z H r (M; Z) Z. r=0 Poincaré-Hopf The Euler class e(m) = e(tm) of the tangent bundle TM of a connected oriented m-dimensional manifold M equals the Euler characteristic of M (= 0 if m is odd) e(m) = χ(m) H m (M; Z) = Z. Can be proved by algebraic topology (e.g. Milnor-Stasheff) or by applying the Index Theorem to the operator D = d + d : Λ odd (T M) Λ even (T M) with analytic index χ(m) and topological index e(m). A generic section of T (M) has χ(m) zeros. TM admits a non-zero section if and only if χ(m) = 0

4 The Stiefel-Whitney, Chern and Pontrjagin classes 7 The Stiefel-Whitney classes of a real bundle V with d k = k w k (V ) H k (M; Z 2 ) (k 0). Depend only on the homotopy type of the sphere bundle. The Chern classes of a complex bundle V with d k = 2k c k (V ) H 2k (M; Z) (k 0). Index theory depends on the construction of Chern classes of complex vector bundles using connections, curvature and de Rham cohomology. The Pontrjagin classes of a real bundle V with d k = 4k p k (V ) = c 2k (V R C) H 4k (M; Z) (k 0). Standard method: define class for k = 0 by convention, for k = 1 by construction, and extend to k 2 by splitting principle and product formula. References: Characteristic classes by Milnor and Stasheff. Differential forms in algebraic topology by Bott and Tu. Some examples of Chern classes From now on, will only consider complex vector bundles V and cohomology with complex coefficients, with Chern classes c k (V ) H 2k (M) = H 2k (M; C). Example The Chern classes of a flat (e.g. trivial) complex bundle V are Will define flat bundles below. c k (V ) = 0 H 2k (M) for k 1. Example The Chern classes of a 1-dimensional (= line) complex bundle V are { e(v R ) for k = 1 c k (V ) = 0 for k 2 H2k (M) with V R = V regarded as a real 2-dimensional oriented bundle. 8

5 Some properties of the Chern classes Splitting principle For every complex m-dimensional vector bundle V over M there exists a map f : N M such that (i) f : H (M) H (N) is injective. (ii) f V = L 1 L 2 L m is a Whitney sum of line bundles L 1, L 2,..., L m. The cup product multiplication in cohomology H i (M) H j (M) H i+j (M) ; x y xy is particularly significant in the study of characteristic classes. Product formula The Chern classes of the Whitney sum of complex vector bundles V, W over M is given by c k (V W ) = c i (V )c j (W ) H 2k (M). i+j=k Naturality, the splitting principle and the product formula show that the Chern classes of all complex vector bundles are determined by the Chern classes of line bundles. 9 The transition functions of a complex vector bundle 10 U(m) = Lie group of m m matrices A = (a jk C) 1 j,k m which are unitary, i.e. preserve the hermitian form on C m : m AA = I, or equivalently a jl a lk = δ jk C. The projection of a complex m-plane vector bundle over M l=1 C m V p M has local trivializations (U M, φ U : p 1 (U) = U C m ) for a covering {U} of M by open subsets U M. For any U 1, U 2 with U 1 U 2 have φ U2 (φ 1 U 1 ) : (U 1 U 2 ) C m (U 1 U 2 ) C m ; (x, y) (x, h 12 (x)(y)) with h 12 (x) U(m) the transition functions.

6 11 Flat bundles I. Definition A complex vector bundle V is flat if the transition functions h 12 : U 1 U 2 U(m) are locally constant. The higher Chern classes of a flat bundle V are trivial c k (V ) = 0 H 2k (M) for k 1. Proposition Let M be a connected manifold with universal cover π 1 (M) M M. A group morphism π 1 (M) U(m) let V determines a flat vector bundle C m V = C m π1 (M) M M. Proof The transition functions are continuous, with h 12 : U 1 U 2 π 1 (M) U(m), and π 1 (M) has the discrete topology. Flat bundles II. Proposition A complex bundle V over a connected M is flat if and only if it is of the type 12 V = C m π1 (M) M for some group morphism π 1 (M) U(m). Example Every complex bundle V over S 1 is flat, being of the form V = C m Z R = (C m [0, 1])/{(z, 0) (u(z), 1) z C m } for some u U(m), with projection V R/Z = S 1 ; [z, t] e 2πit.

7 The first Chern class c 1 of the tautological complex line bundle V n over CP n The n-dimensional complex projective space CP n is the real oriented 2n-dimensional manifold of complex 1-dimensional subspaces L C n+1, with { H r (CP n C if 0 r even 2n ) = 0 otherwise. The tautological complex line bundle over CP n is C V n = {(L, z) z L C n+1 } CP n with S 1 S(V n ) = S 2n+1 CP n the Hopf fibration. c 1 (V n ) 0 and V n is not flat, since in view of the Wang sequence 13 H r (S 2n+1 ) H r (CP n ) e(v R n ) H r 2 (CP n ) H r 1 (S 2n+1 )... (using Z-coefficients) the first Chern class of V n has to be c 1 (V n ) = e(v R n ) = 1 H 2 (CP n ) = C. The Chern-Weil method, following Roe Following the middle section of Chapter 2 of Elliptic operators, topology and asymptotic methods by John Roe, word for word. There are many approaches to the theory of characteristic classes. The one best suited for index theory is the so-called Chern-Weil method, using curvature to define the Chern and Pontrjagin classes with complex coefficients. We will consider characteristic classes with complex coefficients of complex vector bundles. We will represent H (M) = H (M; C) as de Rham cohomology, that is closed forms (the kernel of d) modulo exact forms (the image of d). 14

8 15 Brief review of vector fields, the Lie bracket and connections For a manifold M let C (M) be the ring of functions f : M C. For a complex vector bundle V over M let C (V ) be the C (M)-module of sections M V. A vector field X C (TM) is a smooth section of the tangent bundle TM, or equivalently a derivation X : C (M) C (M) ; f (X (f ) : x df (X (x))). The Lie bracket [X, Y ] C (TM) of vector fields X, Y C (TM) is [X, Y ] : C (M) C (M) ; f X (Y (f )) Y (X (f )). A connection on a complex vector bundle V over M is a linear map : C (TM) C (V ) C (V ) ; X Y X Y with X : C (V ) C (V ) the covariant derivative. Every complex vector bundle V admits connections. Connections, curvature and non-flatness In some sense, the curvature operator K of a connection on a vector bundle V measures the local deviation of V from flatness. If V is flat, and the base manifold M is simply-connected, then V is trivial. This suggests that there may be a link between curvature and characteristic classes, which measure the global deviation of V from triviality. Such a link is provided by the theory of invariant polynomials. 16

9 17 Invariant polynomials I. Let gl m (C) denote the Lie algebra of mxm matrices over C. Definition 2.18A An invariant polynomial on gl m (C) is a polynomial function P : gl m (C) C such that P(XY ) = P(YX ) C for all X, Y gl m (C). Example The determinant and the trace are invariant polynomials. Definition 2.18B An invariant formal power series is a formal power series over gl m (C) each of whose homogeneous components is an invariant polynomial. Lemma 2.19 The ring of invariant polynomials on gl m (C) is a polynomial ring generated by the polynomials c k (X ) = ( 2πi) k tr(λ k X ) where Λ k X denotes the transformation induced by X on Λ k C m Invariant polynomials II. Let P be any invariant polynomial. If we first of all look at the restriction of P to diagonal matrices, we see that P must be a polynomial function of the diagonal entries. Since these diagonal entries can be interchanged by conjugation, P must in fact be a symmetric polynomial function. P is invariant under conjugation, so it is a symmetric polynomial function of the eigenvalues for all matrices with distinct eigenvalues: by linear algebra such matrices are conjugate to diagonal matrices. But the set of such matrices is dense in gl m (C), so a continuity argument shows that P is just a symmetric polynomial function in the eigenvalues. Now it is easy to see that tr(λ k X ) is the kth elementary symmetric function in the eigenvalues of X. The main theorem on symmetric polynomials (Chapter IV of Lang s Algebra) states that the ring of symmetric polynomials is itself a polynomial ring generated by the elementary symmetric functions, and this now completes the proof. 18

10 The invariant polynomial P(K) of the curvature K of a connection The curvature operator K of is the 2-form on M with values in End(V ): if X, Y C (TM) are vector fields and Z C (V ) is a section of V, then K(X, Y )Z = X Y Z Y X Z [X,Y ] Z. Choosing a local framing for V, we may identify K with a matrix of ordinary 2-forms. If P is an invariant polynomial, we may apply P to this matrix to get an even-dimensional differential form P(K). Because of the invariant nature of P, the form P(K) is independent of the choice of local framing, and is therefore globally defined. 19 The invariant polynomial P(K) as a form In terms of the principal GL m (C)-bundle E associated to an m-dimensional complex vector bundle V and the terminology introduced earlier in Chapter 2, this construction may be phrased as follows. Let ω be the gl m (C) valued 1-form of the induced connection on the principal bundle E, and let Ω be the curvature form (2.10) Ω(X 1, X 2 ) = dω(x 1, X 2 ) + [ω(x 1 ), ω(x 2 )]. Ω is a horizontal, equivariant 2-form on E with values in gl m (C), so P(Ω) is a horizontal invariant form on E. Such a form is the lift to E of a form on M, and this form on M is P(K). 20

11 The differential form P(K) I. Whichever approach is adopted, notice that since 2-forms are nilpotent elements in the exterior algebra, all formal power series with 2-form valued variables in fact converge. Thus, the construction makes good sense if P is merely an invariant formal power series. Proposition 2.20 For any invariant polynomial (or formal power series) P, the differential form P(K) is closed, and its de Rham cohomology class is independent of the choice of connection on V. Proof For the purposes of this proof let us describe an invariant formal power series P as respectable if the conclusion of the proposition holds for P. Clearly the sum and product of respectable formal power series are respectable. Thus, it is enough to prove that the generators defined in (2.19) are respectable. Equivalently, since det(1 + qk) = k q k tr(λ k K) it is enough to prove that det(1 + qk), considered as a formal power series depending on the parameter q, is respectable. 21 The differential form P(K) II. If P is a respectable formal power series with constant term a, and g is a function holomorphic about a, then g P is also a respectable formal power series. Hence, det(1 + qk) is respectable if and only if log det(1 + qk) is respectable. We will now prove directly that log det(1 + qk) is respectable. For this purpose we will work in the associated principal bundle E of frames for V, with matrix-valued connection 1-form ω and corresponding curvature 2-form Ω. We will use the formula (2.10) ( ) Ω = dω + ω 2 where the product in the ring of matrix-valued forms is obtained by tensoring exterior product and matrix multiplication. 22

12 The differential form P(K) III. Now suppose that ω depends on a parameter t; then Ω also depends on t, and if we use a dot to denote differentiation with respect to t, then Consider d log det(1 + qω) dt = Ω = d ω + ω ω + ωω. q tr{(1 + qω) 1 Ω} = ( 1) k q k+1 {Ω k (d ω + ω ω + ωω)} k=0 We need the second Bianchi identity dω = Ωω ωω which follows from ( ) on taking the exterior derivative and then substituting back for dω. We have tr{ω k (ω ω + ωω)} 23 = tr{ω k ω ω ωω k ω} (by the symmetry of trace) = tr{(dω k ) ω} (by the Bianchi identity). Therefore so The differential form P(K) IV. tr{ω k ω ω ωω k ω} = dtr{ω k ω}, d dt log det(1 + qω) = d ( 1) k q k+1 tr(ω k ω} is an exact form; in fact it is the exterior derivative of a horizontal and invariant form on E. Therefore, the projection to the base manifold (d/dt) log det(1 + qk) is also exact. Now the result follows: for since any connection can be deformed locally to flatness, we see that log det(1 + qk) is locally exact, that is closed; and since any two connections can be linked by a (differentiable) path, the cohomology class of log det(1 + qk) is independent of the choice of connection. k=0 24

13 25 The Chern classes of a complex vector bundle It follows from Proposition 2.20 that any invariant formal power series P defines a characteristic class for complex vector bundles. by the recipe pick any connection and apply P to the curvature. Definition 2.21 The generators c k defined in 2.19 correspond to characteristic classes called Chern classes. From 2.19, any characteristic class defined by an invariant polynomial is therefore a polynomial in the Chern classes. The Pontrjagin classes of a real vector bundle 26 (2.22) Suppose now that V is a real vector bundle, and let V C denote its complexification, V C = V R C. The odd Chern classes of V C are equal to zero (in complex cohomology). To see this, notice that we can give V a metric and compatible connection. The curvature F of such a connection is skew (o(m)-valued), so tr(λ k F ) = ( 1) k tr(λ k F ). The even Chern classes of V C are called the Pontrjagin classes of V and are denoted p k (V ) = ( 1) k c 2k (V C ) H 4k (M). If V is an oriented even-dimensional real vector bundle over M it also has an extra characteristic class called the Euler class e(v ) H dim R(V ) (M), corresponding to the Pfaffian invariant polynomial. This is discussed in the exercises.

14 Genera I. 27 Holomorphic functions can be used to build important combinations of characteristic classes. In fact, let f (z) be any function holomorphic near z = 0. We can use f to construct an invariant formal power series Π f, by putting Π f (X ) = det( 1 f (X )). 2πi The associated characteristic class is called the Chern f -genus. Notice that the Chern f -genus has the properties (i) for a complex line bundle L Π f (L) = f (c 1 (L)). (ii) for any complex vector bundles V 1 and V 2 Π f (V 1 V 2 ) = Π f (V 1 )Π f (V 2 ) (proved using a direct sum connection). In fact, it can be seen that these two properties determine the characteristic classes uniquely: this follows from the splitting principle. Genera II. If the eigenvalues of the matrix 1 2πi f (X ) are denoted (x j). then Π f (X ) = f (x j ) 28 is a symmetric formal power series in the x j, which can therefore be expressed in terms of the elementary symmetric functions of the x j. But these elementary symmetric functions are just the Chern classes. Thus in the literature the genus Π f (V ) is often written as Π f (V ) = f (x j ) where x 1, x 2,..., x m are formal variables subject to the relations x 1 +x 2 + +x m = c 1, (x 1 ) 2 +(x 2 ) 2 + +(x m ) 2 = c 2 and so on. In terms of the splitting principle, the formal variables x j can be considered to represent the first Chern classes c 1 (L j ) of the line bundles L 1,..., L m in f V = L 1 L m.

15 29 The total Chern class Example 2.23 The total Chern class c(v ) = 1 + c 1 (V ) + c 2 (V ) +... is the genus associated with f (z) = 1 + z. The multiplicative law c(v 1 V 2 ) = c(v 1 )c(v 2 ) is the Whitney sum formula for the Chern classes. Example 2.24 The genus associated with f (z) = (1 + z) 1 l can be worked out by expanding the product (1 + x j ) 1 as (1 x 1 +(x 1 ) 2... )(1 x 2 +(x 2 ) 2... )(... ) = 1 c 1 +((c 1 ) 2 c 2 )+... If V 1 V 2 is trivial, this expresses the Chern classes of V 2 in terms of those of V 1 The Chern character 30 (2.25) The Chern character ch is the characteristic class associated to the formal power series X tr exp( 1 2πi X ). In terms of the formal variables x j introduced above we have ch(v ) = e x j. The Chern character is not a genus in the sense described above, because of the appearance of a sum rather than the product; but it does have the analogous property ch(v 1 V 2 ) = ch(v 1 ) + ch(v 2 ). Moreover, the identity e x 1 e x 2 = e x 1+x 2 implies that ch(v 1 V 2 ) = ch(v 1 )ch(v 2 ). Thus, ch is a kind of ring homomorphism. Direct calculation of the first few terms yields ch(v ) = (dim V ) + c (c2 1 2c 2) +....

16 The Pontrjagin character I. 31 (2.26) There is an analogous theory of genera for real vector bundles. Let g be holomorphic near 0, with g(0) = 1. Let f be the branch of z (g(z 2 )) 1/2 which has f (0) = 1. Notice that f is an even function of z and therefore the associated genus involves only the even Chern classes. By definition, the Pontrjagin g-genus of a real vector bundle V is the Chern f -genus of its complexification V C. The appearance of the various squares and square roots is explained by the following lemma. Lemma 2.27 Let g be as above. Then for a real vector bundle V, the Pontrjagin g-genus is equal to g(y j ) where the Pontrjagin classes of V are the elementary symmetric functions in the formal variables y j j The Pontrjagin character II. 32 Proof Regard this as an identity between invariant polynomials over o(n) (= the Lie algebra of all skew-symmetric real n n matrices). Any matrix in o(n) is similar to one in block ( diagonal ) form, where the blocks 0 λ are 2 2 and are of the form X = with eigenvalues iλ. λ 0 Since both sides of the desired identity are multiplicative for direct sums, it is enough to prove it for this block X. Now c 1 (X ) = 0, c 2 (X ) = 1 (2πi) 2 (iλ)( iλ) = λ2 4π 2. Thus ( y = ) p 1 (X ) = λ 2 /4π 2. On the other hand, X is similar over C to iλ 0 and so 0 iλ Π f (X ) = f ( λ 2π )f ( λ 2π ) = f (λ/2π)2 as required = g(λ 2 /4π 2 ) = g(y)

17 The Â-genus and the L-genus 33 As in the complex case, one can also interpret this lemma in terms of an appropriate splitting principle; one can take a suitable pull-back of V to split as a direct sum of real 2-plane bundles, and the y j are then the first Pontrjagin classes of the summands. Example 2.28 Two important examples are the Â-genus Â(V ), which is the Pontrjagin genus associated to the holomorphic function z/2 z sinh z/2, and the Hirzebruch L-genus L(V ), which is the Pontrjagin genus associated with the holomorphic function z z tanh z. As we will see, these combinations of characteristic classes arise naturally in the proof of the Index Theorem.

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