Jacobi forms and the structure of Donaldson invariants for 4-manifolds with b_+=1

Abstract

We prove structure theorems for the Donaldson invariants of 4-manifolds with b_+=1, similar to those of Kronheimer and Mrowka in the case b_+>1: We show that for a 4-manifold with b_+=1 and two different period points F, G on the boundary of the positive cone, the difference of the Donaldson invariants at F and G satisfies the k^{th}-order simple type condition for a number k, explicitely given in terms of F and G. We give a formula for this difference in terms of a set of basic classes, expressed in a universal formula involving modular forms. The leading terms coincide with the formulas of Kronheimer and Mrowka in the simple type case. The basic classes are related to the Seiberg Witten invariants. The proof of these results is based on the conjecture of Kotschick and Morgan on the structure of the wall-crossing terms. We relate the problem to new theta functions for indefinite lattices.