STACKY GKM GRAPHS AND ORBIFOLD GROMOV-WITTEN THEORY

A smooth GKM stack is a smooth Deligne-Mumford stack equipped with an action of an algebraic torus T, with only finitely many zero-dimensional and one-dimensional orbits. (i) We define the stacky GKM graph of a smooth GKM stack, under the mild assumption that any one-dimensional T-orbit closure contains at least one T fixed point. The stacky GKM graph is a decorated graph which contains enough information to reconstruct the T-equivariant formal neighborhood of the 1-skeleton (union of zero-dimensional and onedimensional T-orbits) as a formal smooth DM stack equipped with a T-action. (ii) We axiomize the definition of a stacky GKM graph and introduce abstract stacky GKM graphs which are more general than stacky GKM graphs of honest smooth GKM stacks. From an abstract GKM graph we construct a formal smooth GKM stack. (iii) We define equivariant orbifold Gromov-Witten invariants of smooth GKM stacks, as well as formal equivariant orbifold Gromov-Witten invariants of formal smooth GKM stacks. These invariants can be computed by virtual localization and depend only the stacky GKM graph or the abstract stacky GKM graph. Formal equivariant orbifold GromovWitten invariants of the stacky GKM graph of a smooth GKM stack X are refinements of equivariant orbifold Gromov-Witten invariants of X.