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Δs2 = Δx2 + Δy2 + Δz2 

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¶ZÂ÷¨ç¼Æ d(x, y) = [ (y2-x2)2 + (y1-x1)2 ](1/2)

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Geometrically, the Ricci curvature is the mathematical object that controls the growth rate of the volume of metric balls in a manifold.

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gij = δij + O(|x|2)

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