Kirwan surjectivity and Lefschetz-Sommese theorems for a generalized hyperkähler reduction

Abstract: Let $G$ be a compact Lie group. We study a class of Hamiltonian $(G \times S^{1})$-manifolds decorated with a function $s$ with certain equivariance properties, under conditions on the $G$-action which we call of (semi-)linear type. In this context, a close analogue of hyperk\"ahler reduction is defined, and our main result establishes surjectivity of an appropriate analogue of Kirwan's map. As a particular case, our setting includes a class of hyperk\"ahler manifolds with trihamiltonian torus actions, to which our surjectivity result applies.