Almost Isotropic Kaehler Manifolds

Abstract

Let $M$ be a complete Riemannian manifold and suppose $p\in M$. For each unit vector $v \in T_p M$, the $\textit{Jacobi operator}$, $\mathcal{J}_v: v^\perp \rightarrow v^\perp$ is the symmetric endomorphism, $\mathcal{J}_v(w) = R(w,v)v$. Then $p$ is an $\textit{isotropic point}$ if there exists a constant $\kappa_p \in \mathbf{R}$ such that $\mathcal{J}_v = \kappa_p \textit{ Id}_{v^\perp}$ for each unit vector $v \in T_pM$. If all points are isotropic, then $M$ is said to be isotropic; it is a classical result of Schur that isotropic manifolds of dimension at least 3 have constant sectional curvatures. In this paper we consider $\textit{almost isotropic manifolds}$, i.e. manifolds having the property that for each $p \in M$, there exists a constant $\kappa_p \in \mathbb{R}$, such that the Jacobi operators $\mathcal{J}_v$ satisfy $\text{rank}(\mathcal{J}_v - \kappa_p \textit{Id}_{v^\perp}) \leq 1$ for each unit vector $v \in T_pM$. Our main theorem classifies the almost isotropic simply connected K\"ahler manifolds, proving that those of dimension $d=2n \geq 4$ are either isometric to complex projective space or complex hyperbolic space or are totally geodesically foliated by leaves isometric to $\mathbf{C}^{n-1}.$