Distler, JacquesNelson, Philip C2023-05-232023-05-2319912017-04-21https://repository.upenn.edu/handle/20.500.14332/43267Talk presented by P.N. at “Mathematics and Physics of Strings,” Berkeley and “Topology and Geometry in Theoretical Physics,” Turku (Finland), 1991.Physical Sciences and MathematicsPhysicsPresentation