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Introduction-Let (My g) be a Riemannian manifold. If a Riemannian metric g* on M is related to g as g* =e 2p g for some smooth function p on M, then g* is said to be conformai to g. This relation is obviously an equivalence relation in the set of all the Riemannian metrics on M.
Morio Obata (Fri,) studied this question.