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Dec15-04, 02:45 PM
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how is it usually described or defined??

i googled this whicha int much:

The Weyl tensor has the special property that it is invariant under conformal changes to the metric. That is, if g' = f g for some positive scalar function then W' = W. For this reason the Weyl tensor is also called the conformal tensor. It follows that a necessary condition for a Riemannian manifold to be conformally flat is that the Weyl tensor vanish. It turns out that in dimensions >= 4 this condition is sufficient as well