The only Kähler manifold with degree of mobility $\ge 3$ is $(CP(n),g_{Fubini-Study})$

Abstract

The degree of mobility of a (pseudo-Riemannian) Kähler metric is the dimension of the space of metrics h-projectively equivalent to it. We prove that a metric on a closed connected manifold can not have the degree of mobility $\ge 3$ unless it is essentially the Fubini-Study metric, or the h-projective equivalence is actually the affine equivalence. As the main application we prove an important special case of the classical conjecture attributed to Obata and Yano, stating that a closed manifold admitting an essential group of h-projective transformations is $(CP(n),g_{Fubini-Study})$ (up to a multiplication of the metric by a constant). An additional result is the generalization of a certain result of Tanno 1978 for the pseudo-Riemannian situation.

31 pages, 2 .eps figures. No essential changes w.r.t. v1 (misprints, reference updated,etc.)

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