Killing Initial Data
  We study space-time Killing vectors in terms of their "lapse and shift" relative to some spacelike slice. We give a necessary and sufficient condition in order for these lapse-shift pairs, which we call Killing initial data (KID'S), to form a Lie algebra under the bracket operation induced by the Lie commutator of vector fields on space-time. This result is applied to obtain a theorem on the periodicity of orbits for a class of Killing vector fields in asymptotically flat space-times. 
R.Beig (Institut f\"ur Theoretische Physik, Universit\"at Wien) Piotr   T. Chrusciel (Departement de Mathematique Faculte de...