A Simons' type formula for spacelike submanifolds in semi-Riemannian warped product
We determine a Simons' type formula for spacelike submanifolds within a broad class of semiRiemannian warped products. This formula extends the Simons' type formulas initially introduced by Nomizu and Smyth in 1969 for constant mean curvature hypersurfaces in space forms. Furthermore, it incorporates the 2013 extension by Fetcu and Rosenberg for submanifolds with parallel mean curvature vector field in product spaces. From a global standpoint, we establish that compact spacelike hypersurfaces with parallel mean curvature and non-negative sectional curvature are isoparametric hypersurfaces. This result constitutes a generalization of the Riemannian case within space forms, as demonstrated by Nomizu and Smyth. From a local standpoint, we extend analogous results previously established by Asperti, Lobos and Mercuri for pseudo-parallel immersions in Riemannian space forms. As consequences, we prove that any semi-parallel spacelike hypersurface with zero mean curvature in the de Sitter spacetime is totally geodesic. In contrast, there is no semi-parallel spacelike hypersurface with zero mean curvature in the Einstein-de Sitter spacetime.