The Scalar curvature reference article from the English Wikipedia on 24-Jul-2004
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Scalar curvature

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In mathematics, the scalar curvature of a surface is the familiar Gaussian curvature. For higher-dimensional manifolds, it is double of the sum of all the sectional curvatures along all the 2-planes spanned by some orthonormal frame. It is also the full trace of the Ricci curvature as well as of the curvature tensor.