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单位球面上具有非负M(o)bius截面曲率的超曲面

         

摘要

Let x : M → Sn+1 be a hypersurface in the (n + 1)-dimensional unit sphere Sn+1 without umbilic point. The M(o)bius invariants of x under the M(o)bius transformation group of Sn+1 are M(o)bius metric, M(o)bius form, M(o)bius second fundamental form and Blaschke tensor. In this paper, we prove the following theorem: Let x: M → Sn+1 (n ≥ 2) be an umbilic free hypersurface in Sn+1with nonnegative M(o)bius sectional curvature and with vanishing M(o)bius form. Then x is locally M(o)bius equivalent to one of the following hypersurfaces: (i) the torus Sk(a) × Sn-k(√1-a2) with 1 ≤ k ≤ n - 1; (ii) the pre-image of the stereographic projection of the standard cylinder Sk × Rn-k (∩) Rn+1 with 1 ≤ k ≤ n - 1; (iii) the pre-image of the stereographic projection of the cone in Rn+1: ~x(u, v, t) = (tu, tv),where (u,v,t) ∈ Sk(a) × Sn-k-1( √1- a2) × R+.

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