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List of formulas in Riemannian geometry
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List of formulas in Riemannian geometry : ウィキペディア英語版
List of formulas in Riemannian geometry
This is a list of formulas encountered in Riemannian geometry.
==Christoffel symbols, covariant derivative==

In a smooth coordinate chart, the Christoffel symbols of the first kind are given by
:\Gamma_=\frac12 \left(
\frac g_
+\frac g_
-\frac g_
\right)
=\frac12 \left( g_ + g_ - g_ \right) \,,

and the Christoffel symbols of the second kind by
:\begin
\Gamma^m\Gamma_\\
&=\frac\, g^ \left(
\frac g_
+\frac g_
-\frac g_
\right)
=\frac\, g^ \left( g_ + g_ - g_ \right) \,.
\end

Here g^ is the inverse matrix to the metric tensor g_. In other words,
:
\delta^i

and thus
:
n = \delta^ig_

is the dimension of the manifold.
Christoffel symbols satisfy the symmetry relations
:\Gamma_ = \Gamma_ or, respectively, \Gamma^i_,
the second of which is equivalent to the torsion-freeness of the Levi-Civita connection.
The contracting relations on the Christoffel symbols are given by
:\Gamma^i g^\frac=\frac \frac = \frac \
and
:g^\Gamma^i\,g^\right)}
where |''g''| is the absolute value of the determinant of the metric tensor g_\ . These are useful when dealing with divergences and Laplacians (see below).
The covariant derivative of a vector field with components v^i is given by:
:
v^i +\Gamma^i=(\nabla_j v)_i=\frac-\Gamma^k this becomes
:
v^=\frac +\Gamma^i+\Gamma^j

and likewise for tensors with more indices.
The covariant derivative of a function (scalar) \phi is just its usual differential:
:
\nabla_i \phi=\phi_=\phi_=\frac

Because the Levi-Civita connection is metric-compatible, the covariant derivatives of metrics vanish,
:
(\nabla_k g)_ = (\nabla_k g)^ = 0

as well as the covariant derivatives of the metric's determinant (and volume element)
:
\nabla_k \sqrt=0

The geodesic X(t) starting at the origin with initial speed v^i has Taylor expansion in the chart:
:
X(t)^i=tv^i-\frac\Gamma^i{}_{jk}v^jv^k+O(t^3)


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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