## Asymptotic symmetry and the global structure of future null infinity

Space-times for which j/sup +/ (future null infinity) is not necessarily homeomorphic to R x S/sup 2/ are considered. It is shown that, depending on the global conformal structure of j/sup +/, a given space-time either (1) possesses an asymptotic symmetry group with a normal subgroup of supertranslations, similar in structure to the BMS group, or (2) possesses a simpler kind of asymptotic symmetry group, not involving supertranslations, or (3) has no asymptotic symmetry. The setting is Newman and Unti's approach to asymptotically flat space-times and use is made of the characterization of the asymptotic symmetry transformation as a conformalmore »