We develop in detail the theory of c-projective geometry, a natural analogue of projective differential geometry adapted to complex manifolds. We realise it as a type of parabolic geometry and describe the associated Cartan or tractor connection. A Kähler manifold gives rise to a c-projective structure and this is one of the primary motivations for its study. The existence of two or more Kähler metrics underlying a given c-projective structure has many ramifications, which we explore in depth. As a consequence of this analysis, we prove the Yano-Obata Conjecture for complete Kähler manifolds: if such a manifold admits a one parameter group of c-projective transformations that are not affine, then it is complex projective space, equipped with a multiple of the Fubini-Study metric.
Proof. By assumption, we haveIf ∇ a q = 0, it follows easily that p and q are locally constant hence constant. Otherwise, contracting this expression with a nonzero tangent vector X a in the kernel of ∇ a q, we deduce that Λ b = qΛ b and ∇ a p = −ξ∇ a q for some function ξ., it follows from what we have already proven that ξ is constant. Otherwise, we deduce that p and q are constant.