Generalized Gauss–Bonnet theorem

In mathematics, the generalized Gauss–Bonnet theorem (also called Chern–Gauss–Bonnet theorem) presents the Euler characteristic of a closed even-dimensional Riemannian manifold as an integral of a certain polynomial derived from its curvature. It is a direct generalization of the Gauss–Bonnet theorem to higher dimensions.

Let M be a compact 2n-dimensional Riemannian manifold without boundary, and let Ω be the curvature form of the Levi-Civita connection. This means that Ω is an $\mathfrak s\mathfrak o(2n)$-valued 2-form on M. So Ω can be regarded as a skew-symmetric 2n × 2n matrix whose entries are 2-forms, so it is a matrix over the commutative ring $\bigwedge^{\hbox{even}}T^*M$. One may therefore take the Pfaffian of Ω, Pf(Ω), which turns out to be a 2n-form.

The generalized Gauss–Bonnet theorem states that

$\int_M \mbox{Pf}(\Omega)=(2\pi)^n\chi(M)\$

where χ(M) denotes the Euler characteristic of M.

Example: dimension 4

In dimension n = 4, for a compact oriented manifold, we get

$\chi(M)=\frac{1}{8\pi^2}\int_M\left(|Rm|^2-4|Rc|^2+R^2\right)d\mu$

where Rm is the full Riemann curvature tensor, Rc is the Ricci curvature tensor, and R is the scalar curvature.

Further generalizations

As with the two-dimensional Gauss–Bonnet Theorem, there are generalizations when M is a manifold with boundary.

The Gauss–Bonnet Theorem can be seen as a special instance in the theory of characteristic classes. The Gauss–Bonnet integrand is the Euler class. Since it is a top-dimensional differential form, it is closed. The naturality of the Euler class means that when you change the Riemannian metric, you stay in the same cohomology class. That means that the integral of the Euler class remains constant as you vary the metric, and so is an invariant of smooth structure.

An extremely far-reaching generalization of the Gauss–Bonnet Theorem is the Atiyah–Singer Index Theorem. Let D be a (weakly) elliptic differential operator between vector bundles. That means that the principal symbol is an isomorphism. (Strong ellipticity would furthermore require the symbol to be positive-definite.) Let D * be the adjoint operator. Then the index is defined as dim(ker(D))-dim(ker(D*)), and by ellipticity is always finite. The Index Theorem states that this analytical index is constant as you vary the elliptic operator smoothly. It is in fact equal to a topological index, which can be expressed in terms of characteristic classes. The 2-dimensional Gauss–Bonnet Theorem arises as the special case where the analytical index is defined in terms of Betti numbers and the topological index is defined in terms of the Gauss–Bonnet integrand.

See also

• Chern–Weil homomorphism
• Pontryagin number
• Pontryagin class

References

• Cycon, Hans; Froese, Richard; Kirsch, Werner; Simon, Barry (1987), Schroedinger operators (1st ed.), Berlin, New York: Springer-Verlag, ISBN 3-540-16758-7  Chapter 12
• Chern, Shiing-Shen (1945), "On the curvatura integra in Riemannian manifold", Annals of Mathematics 48 (4): 674–684, JSTOR 1969203  This is the historically first time that Chern-Gauss-Bonnet was proven without assuming the manifold to be a hypersurface. For hypersurfaces, the result had been shown first by Allendoerfer and Weil in 1940 which is cited in this paper of Chern.