In mathematics, the Chern–Weil homomorphism is a basic construction in the Chern–Weil theory that computes topological invariants of vector bundles and principal bundles on a smooth manifold M in terms of connections and curvature representing classes in the de Rham cohomology rings of M. That is, the theory forms a bridge between the areas of algebraic topology and differential geometry. It was developed in the late 1940s by Shiing-Shen Chern and André Weil, in the wake of proofs of the generalized Gauss–Bonnet theorem. This theory was an important step in the theory of characteristic classes.
Let G be a real or complex Lie group with Lie algebra ; and let denote the algebra of -valued polynomials on (exactly the same argument works if we used instead of .) Let be the subalgebra of fixed points in under the adjoint action of G; that is, it consists of all polynomials f such that for any g in G and x in ,
where on the right cohomology is de Rham cohomology. Such a homomorphism exists and is uniquely defined for every principal G-bundle P on M. If G is compact, then under the homomorphism, the cohomology ring of the classifying space for G-bundles BG is isomorphic to the algebra of invariant polynomials:
(The cohomology ring of BG can still be given in the de Rham sense:
Definition of the homomorphism
Choose any connection form ω in P, and let Ω be the associated curvature 2-form; i.e., Ω = Dω, the exterior covariant derivative of ω. If is a homogeneous polynomial function of degree k; i.e., for any complex number a and x in , then, viewing f as a symmetric multilinear functional on (see the ring of polynomial functions), let
be the (scalar-valued) 2k-form on P given by
If, moreover, f is invariant; i.e., , then one can show that is a closed form, it descends to a unique form on M and that the de Rham cohomology class of the form is independent of ω. First, that is a closed form follows from the next two lemmas:
- Lemma 1: The form on P descends to a (unique) form on M; i.e., there is a form on M that pulls-back to .
- Lemma 2: If a form φ on P descends to a form on M, then dφ = Dφ.
Indeed, Bianchi's second identity says and, since D is a graded derivation, Finally, Lemma 1 says satisfies the hypothesis of Lemma 2.
To see Lemma 2, let be the projection and h be the projection of onto the horizontal subspace. Then Lemma 2 is a consequence of the fact that (the kernel of is precisely the vertical subspace.) As for Lemma 1, first note
Next, we show that the de Rham cohomology class of on M is independent of a choice of connection. Let be arbitrary connection forms on P and let be the projection. Put
where t is a smooth function on given by . Let be the curvature forms of . Let be the inclusions. Then is homotopic to . Thus, and belong to the same de Rham cohomology class by the homotopy invariance of de Rham cohomology. Finally, by naturality and by uniqueness of descending,
The construction thus gives the linear map: (cf. Lemma 1)
In fact, one can check that the map thus obtained:
is an algebra homomorphism.
Example: Chern classes and Chern character
Let and its Lie algebra. For each x in , we can consider its characteristic polynomial in t:
where i is the square root of -1. Then are invariant polynomials on , since the left-hand side of the equation is. The k-th Chern class of a smooth complex-vector bundle E of rank n on a manifold M:
is given as the image of fk under the Chern–Weil homomorphism defined by E (or more precisely the frame bundle of E). If t = 1, then is an invariant polynomial. The total Chern class of E is the image of this polynomial; that is,
Directly from the definition, one can show cj, c given above satisfy the axioms of Chern classes. For example, for the Whitney sum formula, we consider
where we wrote Ω for the curvature 2-form on M of the vector bundle E (so it is the descendent of the curvature form on the frame bundle of E). The Chern–Weil homomorphism is the same if one uses this Ω. Now, suppose E is a direct sum of vector bundles Ei's and Ωi the curvature form of Ei so that, in the matrix term, Ω is the block diagonal matrix with ΩI's on the diagonal. Then, since , we have:
where on the right the multiplication is that of a cohomology ring: cup product. For the normalization property, one computes the first Chern class of the complex projective line; see Chern class#Example: the complex tangent bundle of the Riemann sphere.
Since , we also have:
Finally, the Chern character of E is given by
where Ω is the curvature form of some connection on E (since Ω is nilpotent, it is a polynomial in Ω.) Then ch is a ring homomorphism:
Now suppose, in some ring R containing the cohomology ring H(M, C), there is the factorization of the polynomial in t:
Example: Pontrjagin classes
If E is a smooth real vector bundle on a manifold M, then the k-th Pontrjagin class of E is given as:
where we wrote for the complexification of E. Equivalently, it is the image under the Chern–Weil homomorphism of the invariant polynomial on given by:
The homomorphism for holomorphic vector bundles
Let E be a holomorphic (complex-)vector bundle on a complex manifold M. The curvature form Ω of E, with respect to some hermitian metric, is not just a 2-form, but is in fact a (1, 1)-form (see holomorphic vector bundle#Hermitian metrics on a holomorphic vector bundle). Hence, the Chern–Weil homomorphism assumes the form: with ,
- The argument for the independent of a choice of connection here is taken from: Akhil Mathew, Notes on Kodaira vanishing . Kobayashi-Nomizu, the main reference, gives a more concrete argument.
- Editorial note: This definition is consistent with the reference except we have t, which is t -1 there. Our choice seems more standard and is consistent with our "Chern class" article.
- Proof: By definition, . Now compute the square of using Leibniz's rule.
- Shiing-Shen Chern, Complex Manifolds Without Potential Theory (Springer-Verlag Press, 1995) ISBN 0-387-90422-0, ISBN 3-540-90422-0.
- The appendix of this book: "Geometry of Characteristic Classes" is a very neat and profound introduction to the development of the ideas of characteristic classes.