2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/78907Recent work by physicists on gravity in two dimensions has a natural generalization to four dimensions, formulated in terms of an analogue of Segal's category [defined for the study of conformal field theory].A talk at the 1999 Utrecht Conference on operadsQuantum AlgebraMathematical PhysicsAlgebraic GeometryAlgebraic TopologyCategory TheoryDifferential Geometry14H10, 53C80,55N22text