Cohn-Vossen's inequality

Short description: Relates the integral of Gaussian curvature of surfaces to the Euler characteristic

In differential geometry, Cohn-Vossen's inequality, named after Stefan Cohn-Vossen, relates the integral of Gaussian curvature of a non-compact surface to the Euler characteristic. It is akin to the Gauss–Bonnet theorem for a compact surface.

A divergent path within a Riemannian manifold is a smooth curve in the manifold that is not contained within any compact subset of the manifold. A complete manifold is one in which every divergent path has infinite length with respect to the Riemannian metric on the manifold. Cohn-Vossen's inequality states that in every complete Riemannian 2-manifold S with finite total curvature and finite Euler characteristic, we have

[math]\displaystyle{ \iint_S K \, dA \le 2\pi\chi(S), }[/math]

where K is the Gaussian curvature, dA is the element of area, and χ is the Euler characteristic.

Examples

• If S is a compact surface (without boundary), then the inequality is an equality by the usual Gauss–Bonnet theorem for compact manifolds.
• If S has a boundary, then the Gauss–Bonnet theorem gives

[math]\displaystyle{ \iint_S K\, dA = 2\pi\chi(S) - \int_{\partial S}k_g\,ds }[/math]

where [math]\displaystyle{ k_g }[/math] is the geodesic curvature of the boundary, and its integral the total curvature which is necessarily positive for a boundary curve, and the inequality is strict. (A similar result holds when the boundary of S is piecewise smooth.)

• If S is the plane R², then the curvature of S is zero, and χ(S) = 1, so the inequality is strict: 0 < 2π.

Notes and references

• Cohn-Vossen, Stefan (1935). "Kürzeste Wege und Totalkrümmung auf Flächen". Compositio Mathematica 2: 69–13. https://eudml.org/doc/88615. 
• Huber, Alfred (1957). "On subharmonic functions and differential geometry in the large". Commentarii Mathematici Helvetici 32 (1): 13–72. doi:10.1007/BF02564570. https://eudml.org/doc/139145. 
• Li, Peter (2000). "Curvature and function theory on Riemannian manifolds". Surveys in Differential Geometry. 7. Somerville, MA: International Press. pp. 375–432. doi:10.4310/SDG.2002.v7.n1.a13. ISBN 1-57146-069-1. 
• Shiohama, Katsuhiro; Shioya, Takashi; Tanaka, Minoru (2003). The geometry of total curvature on complete open surfaces. Cambridge Tracts in Mathematics. 159. Cambridge: Cambridge University Press. doi:10.1017/CBO9780511543159. ISBN 0-521-45054-3. 

External links

• Gauss–Bonnet theorem, in the Encyclopedia of Mathematics, including a brief account of Cohn-Vossen's inequality