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A diffeomorphism of pseudo-Riemannian manifolds is called sectional curvaturepreserving if it preserves the sectional curvature of all the nondegenerate2-planes. We consider a similar condition for degenerate 2-planes and we provethat the diffeomorphism is conformal (when the condition is fulfilled forweakly degenerate planes) or an isometry (when the condition is fulfilled forstrongly degenerate 2-planes).

Diffeomorphisms of pseudo-Riemannian manifolds and the values of the curvature tensor on degenerate planes