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Answers for “光滑流形上黎曼度量的正式定义是什么?”

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A Riemannian metric gg on a smooth manifold MM is a smooth covariant 2-tensor field such that for each point p∈Mp \in M, the value gpg_p is an inner product on the tangent space TpMT_pM. It is symmetric and positive definite.

Conditions: MM is a smooth manifold.; gg is a smooth covariant 2-tensor field.; For each p∈Mp \in M, gpg_p is an inner product on TpMT_pM.; gp(v,v)≥0g_p(v,v) \ge 0 for all v∈TpMv \in T_pM.; gp(v,v)=0g_p(v,v) = 0 if and only if v=0v = 0.

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A covariant 2-tensor field is linear over the ring of smooth functions. If X,YX, Y are vector fields and f,hf, h are smooth functions, then g(fX,hY)=fhg(X,Y)g(fX, hY) = f h g(X,Y).

Conditions: gg is a covariant 2-tensor field.; X,YX, Y are vector fields.; f,hf, h are smooth functions.

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A covariant tensor field takes vectors as inputs and outputs a scalar number. In the context of a Riemannian metric, it operates on pairs of vectors from the tangent space at a given point.

Conditions: The tensor field is covariant.; Inputs are vectors from the tangent space.; Output is a scalar.

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The tangent bundle TMTM of a manifold MM is defined as the disjoint union of all tangent spaces TpMT_pM for every point pp in MM. Symbolically, TM=⨆p∈MTpMTM = \bigsqcup_{p \in M} T_pM.

Conditions: MM is a smooth manifold.; TpMT_pM is the tangent space at point pp.