  In its Euclidean formulation, the AdS/CFT correspondence begins as a study of
Yang-Mills conformal field theories on the sphere, S^4. It has been
successfully extended, however, to S^1 X S^3 and to the torus T^4. It is
natural to hope that it can be made to work for any manifold on which it is
possible to define a stable Yang-Mills conformal field theory. We consider a
possible classification of such manifolds, and show how to deal with the most
obvious objection : the existence of manifolds which cannot be represented as
boundaries. We confirm Witten's suggestion that this can be done with the help
of a brane in the bulk.
