1. Albert-Einstein-Institut, Max-Planck-Institut für Gravitationsphysik, Golm, Germany
2. Institutionen för Matematik, Kungliga Tekniska Högskolan, Stockholm, Sweden
3. Department of Mathematics, University of Miami, Coral Gables, Florida, U.S.A.
4. Department of Mathematics, University of Washington, Seattle, Wa., U.S.A.