Abstract. We prove a formula computing the Gromov-Witten invariants of genus zero with three marked points of the resolution of the transversal A3-singularity of the weighted projective space P(1, 3, 4, 4) using the theory of deformations of surfaces with An-singularities. We use this result to check Ruan's conjecture for the stack P(1, 3, 4, 4).
§1. IntroductionThe main results of this paper concern the weighted projective space P(1, 3, 4, 4). The singular locus of its coarse moduli space |P(1, 3, 4, 4)| is the disjoint union of an isolated singularity of type (1/3)(1, 1, 1) (we use Reid's notation in [17]) and a transversal A 3 -singularity. Using toric methods, we construct a crepant resolution Z of |P(1, 3, 4, 4)|. In Theorem 3.3.1, we determine a formula for certain Gromov-Witten invariants of Z over the A 3 -singularity using the theory of deformations of surfaces with rational double points and the deformation invariance property of Gromov-Witten invariants. We then apply this result in Theorem 5.2.1 to construct a ring isomorphism-predicted in [18] by Ruan's cohomological crepant resolution conjecture -between the quantum corrected cohomology ring of Z and the Chen-Ruan orbifold cohomology of P(1, 3, 4, 4), after evaluating the quantum parameters related to the transversal A 3 -singularity to a fourth root of the unity and putting the last parameter to zero. This last evaluation is quite surprising (in [2], we show that this parameter can be evaluated to 1). To confirm this property, we show in Proposition 5.3.1 that, for all weighted projective spaces P(1, . . . , 1, n) with n weights equal to 1 which have only