Abstract
We study orientable hypersurfaces in a Sasakian manifold. The structure vector field xi of a Sasakian manifold determines a vector field v on a hypersurface that is the component of the Reeb vector field xi tangential to the hypersurface, and it also gives rise to a smooth function sigma on the hypersurface, namely the projection of xi on the unit normal vector field N. Moreover, we have a second vector field tangent to the hypersurface, given by u = -phi(N). In this paper, we first find a necessary and sufficient condition for a compact orientable hypersurface to be totally umbilical. Then, with the assumption that the vector field u is an eigenvector of the Laplace operator, we find a necessary condition for a compact orientable hypersurface to be isometric to a sphere. It is shown that the converse of this result holds, provided that the Sasakian manifold is the odd dimensional sphere S2n+1. Similar results are obtained for the vector field v under the hypothesis that this is an eigenvector of the Laplace operator. Also, we use a bound on the integral of the Ricci curvature Ric(u, u)) $\end{document} of the compact hypersurface to find a necessary condition for the hypersurface to be isometric to a sphere. We show that this condition is also sufficient if the Sasakian manifold is S2n+1.