Homotopical and topological rigidity of hypersurfaces of spherical space forms

Abstract

The first main result is a topological rigidity theorem for complete immersed hypersurfaces of spherical space forms which extends similar results due to do Carmo/Warner, Wang/Xia and Longa/Ripoll. Under certain sharp conditions on the principal curvatures of such a hypersurface $ f \colon N^n \to M^{n+1} $ $( n\ge 2 )$, it asserts that the universal cover of $ N $ must be diffeomorphic to the $ n $-sphere $ {S}^n $, and provides an upper bound for the order of the fundamental group of $ N $ in terms of that of $ M $. In particular, if $ M = {S}^{n+1} $, then $ N $ is diffeomorphic to $ {S}^n $ and either $ f $ or its Gauss map is an embedding. Let $ J \subset (0,\pi) $ be any interval of length less than $ \frac{\pi}{2} $. The second main result constructs a weak homotopy equivalence between the space of all complete immersed hypersurfaces of $ M $ with principal curvatures in $ \cot (J) $ and the twisted product of $ \big( \Gamma\backslash \mathrm{SO}_{n+2} \big) $ and $ \mathrm{Diff}_+({S}^n) $ by $ \mathrm{SO}_{n+1} $, where $ \Gamma $ is the fundamental group of $ M $ regarded as a subgroup of $ \mathrm{SO}_{n+2} $. Relying on another rigidity criterion due to Wang/Xia, the third main result constructs a homotopy equivalence between the space of all complete immersed hypersurfaces of $ S^{n+1} $ whose Gauss maps have image contained in a strictly convex ball and the same twisted product, with $ \Gamma $ the trivial group.