On real bisectional curvature for Hermitian manifolds

Abstract

Motivated by the recent work of Wu and Yau on the ampleness of a canonical line bundle for projective manifolds with negative holomorphic sectional curvature, we introduce a new curvature notion called real bisectional curvature for Hermitian manifolds. When the metric is Kähler, this is just the holomorphic sectional curvature $H$, and when the metric is non-Kähler, it is slightly stronger than $H$. We classify compact Hermitian manifolds with constant nonzero real bisectional curvature, and also slightly extend Wu and Yau’s theorem to the Hermitian case. The underlying reason for the extension is that the Schwarz lemma of Wu and Yau works the same when the target metric is only Hermitian but has nonpositive real bisectional curvature.