Generalized normal homogeneous Riemannian metrics on spheres and projective spaces

Abstract

In this paper we develop new methods of study of generalized normal homogeneous Riemannian manifolds. In particular, we obtain a complete classification of generalized normal homogeneous Riemannian metrics on spheres. We prove that for any connected (almost effective) transitive on $S^n$ compact Lie group $G$, the family of $G$-invariant Riemannian metrics on $S^n$ contains generalized normal homogeneous but not normal homogeneous metrics if and only if this family depends on more than one parameters. Any such family (that exists only for $n=2k+1$) contains a metric $g_{\can}$ of constant sectional curvature 1 on $S^n$. We also prove that $(S^{2k+1}, g_{\can})$ is Clifford-Wolf homogeneous, and therefore generalized normal homogeneous, with respect to $G$ (excepting the groups $G=SU(k+1)$ with odd $k+1$). The space of unit Killing vector fields on $(S^{2k+1}, g_{\can})$ from Lie algebra $\mathfrak{g}$ of Lie group $G$ is described as some symmetric space (excepting the case $G=U(k+1)$ when one obtains the union of all complex Grassmannians in ${\mathbb{C}}^{k+1}$).

32 pages

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