By means of a certain conformal covariant differentiation process we constr
uct generating systems for conformally invariant tensors in a pseudo-Rieman
nian manifold and use these invariants to derive necessary conditions for t
he validity of Huygens' principle: of some conformally invariant field equa
tions as well as for a space-time to be conformally related to an Einstein
space-time. (C) Elsevier, Paris.