Keywords. Lorentz manifold, twisted Heisenberg group, condition (*)
AMS subject classification. Primary: 57B30. Secondary: 57S20.
Submitted: 15 November 1997. Published: 29 October 1998.
Ecole Normale Superieure de Lyon
UMPA, UMR 128 CNRS
46 Allee d'Italie, 69364 Lyon, FRANCE
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