Some Optimal Inequalities for Anti-invariant Submanifolds of the Unit Sphere

In this paper, we study the rigidity phenomena on the (n+1)-dimensional anti-invariant submanifolds of the unit sphere of dimension (2n+ 1) from the intrinsic and extrinsic aspects, respectively. First of all, we establish a basic inequality for such submanifolds relative to the norm of the covariant differentiation of both the second fundamental form h and mean curvature vector field H. Secondly, the lower bound of the norm of H is further derived by means of a general inequality. Finally, in dealing with those minimal anti-invariant submanifolds with eta-Einstein induced metrics, we obtain an inequality in terms of the Weyl curvature tensor, squared norm S of h, and scalar curvature. In particular, these inequalities above are optimal in the sense that all the submanifolds attaining the equalities are completely determined.