Conformal blocks for AdS5 singletons

We give a simple derivation of the conformal blocks of the singleton sector of compactifications of IIB string theory on spacetimes of the form X 5 × Y 5 with Y 5 compact, while X 5 has as conformal boundary an arbitrary 4-manifold M 4 . We retain the second-derivative terms in the action for the B, C fields and thus the analysis is not purely topological. The unit-normalized conformal blocks agree exactly with the quantum partition function of the U (1) gauge theory on the conformal boundary. We reproduce the action of the magnetic translation group and the SL (2 , Z ) S -duality group obtained from the purely topological analysis of Witten. An interesting subtlety in the normalization of the IIB Chern-Simons phase is noted.