“…In this case, the celebrated Crepant Transformation Conjecture of Y. Ruan predicts that the quantum (orbifold) cohomology algebras of X + and X − should be related by analytic continuation in the quantum parameters. This conjecture has stimulated a great deal of interest in the connections between quantum cohomology (or Gromov-Witten theory) and birational geometry: see, for example, [9,10,18,19,21,23,24,28,41,45,53,[56][57][58][59]62,68,71,75,76]. Ruan's original conjecture was subsequently refined, revised, and extended to higher genus Gromov-Witten invariants, first by Bryan-Graber [20] under some additional hypotheses, and then by Coates-Iritani-Tseng, Iritani, and Ruan in general [34,35,50].…”