Caracterizações da esfera em formas espaciais

Abstract

In this work we present three characterizations of the sphere. Initially, it will be shown that given a compact and oriented hypersurface Mn e x: M → Q^(n+1)_c a isometric immersion, x(M) is a geodesic sphere in Q^n+1_c if, and only if, Hr+1 is a nonzero constant and the set of points that are omitted in Qn+1 c by the totally geodesic hypersurfaces (Q^n_c)p tangent to x(M) is non-empty. As a second result, let M be an orientable compact and connected hypersurface with non-negative support function of the Euclidean space Rn+1 and Minkowski's integrand . We prove that the mean curvature function of the hypersurface M is the solution of the Poisson equation = if, and only if, M is isometric to the n-sphere Sn(c) of constant curvature c. similar characterization is proved for a hypersurface with the scalar curvature satisfying the same equation. For the third result we consider an isometric immersion x : M ! Qn+1, where M is a compact hypersurface such that x(M) is convex, and it will be proved that if any r-mean curvature is such that Hr 6= 0 and there are nonnegative constants C1;C2; :::;Cr􀀀1 such that Hr = Pr􀀀1 i=1 CiHi; then x(M) is a geodesic sphere, where Qn+1 is Rn+1, Hn+1 or Sn+1 + .