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A basic inequality and new characterization of Whitney spheres in a complex space form
Authors:Email author" target="_blank">Haizhong?LiEmail author  Luc?Vrancken
Institution:(1) Department of Mathematical Sciences, Tsinghua University, 100084 Beijing, People's Republic, of China;(2) LAMATH, ISTV 2, Campus du Mont Houy, Université de Valenciennes, 59313 Valenciennes Cedex 9, France
Abstract:LetN n (4c) be ann-dimensional complex space form of constant holomorphic sectional curvature 4c and letx:M n N n (4c) be ann-dimensional Lagrangian submanifold inN n (4c). We prove that the following inequality always hold onM n: 
$$\left| {\bar \nabla h} \right|^2  \geqslant \frac{{3n^2 }}{{n + 2}}\left| {\nabla ^ \bot  \vec H} \right|^2 $$
whereh is the second fundamental form andH is the mean curvature of the submanifold. We classify all submanifolds which at every point realize the equality in the above inequality. As a direct consequence of our Theorem, we give, a new characterization of theWhitney spheres in a complex space form. Partially supported by a research fellowship of the Alexander von Humboldt Stiftung.
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