Special metrics in hypercomplex geometry

Abstract: We provide a detailed treatment of special hyperhermitian metrics on hypercomplex manifolds. The quaternionic Gauduchon and quaternionic balanced conditions are investigated at length: we describe their properties and characterize their existence. We also consider strong HKT metrics proving an incompatibility result with balanced hyperhermitian metrics. Additionally, we introduce an Einstein-type condition and determine basic properties and obstructions. Finally, we collect several examples and counterexamples regarding the existence of these types of special metrics as well as two constructions that allow to build new examples.