Abstract
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.