The symplectic sum formula for Gromov–Witten invariants

In the symplectic category there is a 'connect sum' operation that glues symplectic manifolds by identifying neighborhoods of embedded codimension two submanifolds.This paper establishes a formula for the Gromov-Witten invariants of a symplectic sum Z = X#Y in terms of the relative GW invariants of X and Y .Several applications to enumerative geometry are given.Gromov-Witten invariants are counts of holomorphic maps into symplectic manifolds.To define them on a symplectic manifold (X, ω) one introduces an almost complex structure J compatible with the symplectic form ω and forms the moduli space of J-holomorphic maps from complex curves into X and the compactified moduli space, called the space of stable maps.One then imposes constraints on the stable maps, requiring the domain to have a certain form and the image to pass through fixed homology cycles in X.When the correct number of constraints is imposed there are only finitely many maps satisfying the constraints; the (oriented) count of these is the corresponding GW invariant.For complex algebraic manifolds these symplectic invariants can also be defined by algebraic geometry, and in important cases the invariants are the same as the curve counts that are the subject of classical enumerative algebraic geometry.In the past decade the foundations for this theory were laid and the invariants were used to solve several long-outstanding problems.The focus now is on finding effective ways of computing the invariants.One useful technique is the method of 'splitting the domain', in which one localizes the invariant to the set of maps whose domain curves have two irreducible components with the constraints distributed between them.This produces recursion relations relating the desired GW invariant to invariants with lower degree or genus.This paper establishes a general formula describing the behavior of GW invariants under the analogous operation of 'splitting the target'.Because we