Schur's lemma (Riemannian geometry)
= Schur's lemma
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On a connected <Riemannian manifold> of dimension at least three, pointwise independence of <sectional curvature> from the tangent two-plane forces the curvature to be constant. The contracted <Bianchi identity> differentiates the relation $\operatorname{Ric}=(n-1)Kg$ and gives $(n-2)dK=0$.