  We compute the fundamental correlation functions in two-dimensional rational
conformal field theory, from which all other correlators can be obtained by
sewing: the correlators of three bulk fields on the sphere, one bulk and one
boundary field on the disk, three boundary fields on the disk, and one bulk
field on the cross cap. We also consider conformal defects and calculate the
correlators of three defect fields on the sphere and of one defect field on the
cross cap.
  Each of these correlators is presented as the product of a structure constant
and the appropriate conformal two- or three-point block. The structure
constants are expressed as invariants of ribbon graphs in three-manifolds.
