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Shortening embedded curves. (English) Zbl 0686.53036
Let F be a surface. We consider the evolution equation for a map C(u,t): \(S^ 1\times 1\to F\) given by \[ (*)\quad \partial C/\partial t=kN,\quad C(u,0)=c(u), \] where k is the curvature of C and N is its unit normal vector. Now suppose F is convex at infinity, namely the convex hull of every compact subset is compact. It is proved that there exists a solution for (*) on \([0,t_{\infty})\). If \(t_{\infty}\) is finite, then C converges to a point. If \(t_{\infty}\) is infinite, then the curvature of C converges to zero in the \(C^{\infty}\) norm. As an application, it is proved that a 2-sphere with a smooth Riemannian metric has at least three simple closed geodesics.
Reviewer: T.Ochiai

53C20 Global Riemannian geometry, including pinching
58J35 Heat and other parabolic equation methods for PDEs on manifolds
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