Uniqueness of optimal symplectic connections

Abstract

Consider a holomorphic submersion between compact K\"ahler manifolds, such that each fibre admits a constant scalar curvature K\"ahler metric. When the fibres admit continuous automorphisms, a choice of fibrewise constant scalar curvature K\"ahler metric is not unique. An optimal symplectic connection is choice of fibrewise constant scalar curvature K\"ahler metric satisfying a geometric partial differential equation. The condition generalises the Hermite-Einstein condition for a holomorphic vector bundle, through the induced fibrewise Fubini-Study metric on the associated projectivisation. We prove various foundational analytic results concerning optimal symplectic connections. Our main result proves that optimal symplectic connections are unique, up to the action of the automorphism group of the submersion, when they exist. Thus optimal symplectic connections are canonical relatively K\"ahler metrics when they exist. In addition, we show that the existence of an optimal symplectic connection forces the automorphism group of the submersion to be reductive, and that an optimal symplectic connection is automatically invariant under a maximal compact subgroup of this automorphism group. We also prove that when a submersion admits an optimal symplectic connection, it achieves the absolute minimum of a natural log norm functional which we define.