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Tetrad (index notation) : ウィキペディア英語版
Tetrad (index notation)

In Riemannian geometry, we can introduce a coordinate system over the Riemannian manifold (at least, over a chart), giving ''n'' coordinates
:x_\;\text\qquad i = 1, \dots, n
for an n-dimensional manifold. Locally, at least, this gives a basis for the 1-forms, dxi where d is the exterior derivative. The dual basis for the tangent space T is ei.
Now, let's choose an orthonormal basis for the fibers of T. The rest is index manipulation.
==Example==

Take a 3-sphere with the radius ''R'' and give it polar coordinates α, θ, φ.
:e(eα)/R,
:e(eθ)/R sin(α) and
:e(eφ)/R sin(α) sin(θ)
form an orthonormal basis of T.
Call these e1, e2 and e3. Given the metric η, we can ignore the covariant and contravariant distinction for T.
Then, the dreibein (triad),
:e_1 = R\, d\alpha
:e_2 = R\, \sin d\theta
:e_3 = R\, \sin \sin d\phi.
So,
:de_1=0
:de_2=R \cos d\alpha \wedge d\theta
:de_3=R (\cos \sin d\alpha \wedge d\phi + \sin \cos d\theta \wedge d\phi).
from the relation
:d_\mathbf e = de + A \wedge e = 0,
we get
:A_ = -\cos \, d\theta
:A_ = -\cos \, \sin d\phi
:A_ = -\cos \, d\phi.
(dAη=0 tells us A is antisymmetric)
So, \mathbf = d\mathbf + \mathbf \wedge \mathbf,
:F_=\sin d\alpha\wedge d\theta
:F_=\sin \sin d\alpha\wedge d\phi
:F_=\sin^2 \sin d\theta\wedge d\phi


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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