Projective Lichnerowicz-Obata conjecture
arXiv:math/0407337
Abstract
We solve two classical conjectures by showing that if an action of a connected Lie group on a complete Riemannian manifold preserves the geodesics (considered as unparameterized curves), then the metric has constant positive sectional curvature, or the group acts by affine transformations.
38 pages, 1 figure