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Spin-weighted spherical harmonics : ウィキペディア英語版
Spin-weighted spherical harmonics

In special functions, a topic in mathematics, spin-weighted spherical harmonics are generalizations of the standard spherical harmonics and—like the usual spherical harmonics—are functions on the sphere. Unlike ordinary spherical harmonics, the spin-weighted harmonics are U(1) gauge fields rather than scalar fields: mathematically, they take values in a complex line bundle. The spin-weighted harmonics are organized by degree ℓ, just like ordinary spherical harmonics, but have an additional spin weight ''s'' that reflects the additional ''U''(1) symmetry. A special basis of harmonics can be derived from the Laplace spherical harmonics Y_, and are typically denoted by _0Y_ = Y_\ .
Spaces of spin-weighted spherical harmonics were first identified in connection with the representation theory of the Lorentz group . They were subsequently and independently rediscovered by and applied to describe gravitational radiation, and again by as so-called "monopole harmonics" in the study of Dirac monopoles.
==Spin-weighted functions==
Regard the sphere ''S''2 as embedded into the three-dimensional Euclidean space R3. At a point x on the sphere, a positively oriented orthonormal basis of tangent vectors at x is a pair a, b of vectors such that
:
\begin
\mathbf\cdot\mathbf &= \mathbf\cdot\mathbf = 0\\
\mathbf\cdot\mathbf &= \mathbf\cdot\mathbf=1\\
\mathbf\cdot\mathbf &= 0\\
\mathbf\cdot (\mathbf\times\mathbf) &> 0,
\end

where the first pair of equations states that a and b are tangent at x, the second pair states that a and b are unit vectors, the penultimate equation that a and b are orthogonal, and the final equation that (x, a, b) is a right-handed basis of R3.
A spin-weight ''s'' function ''f'' is a function accepting as input a point x of ''S''2 and a positively oriented orthonormal basis of tangent vectors at x, such that
:f(\mathbf x,\cos\theta\mathbf-\sin\theta\mathbf, \sin\theta\mathbf + \cos\theta\mathbf) = e^f(\mathbf x,\mathbf,\mathbf)
for every rotation angle θ.
Following , denote the collection of all spin-weight ''s'' functions by B(''s''). Concretely, these are understood as functions ''f'' on C2\ satisfying the following homogeneity law under complex scaling
:f(\lambda z,\overline\bar) = \left(\frac\right)^s f(z,\bar).
This makes sense provided ''s'' is a half-integer.
Abstractly, B(''s'') is isomorphic to the smooth vector bundle underlying the antiholomorphic vector bundle \overline of the Serre twist on the complex projective line CP1. A section of the latter bundle is a function ''g'' on C2\ satisfying
:g(\lambda z,\overline\bar) = \overline^ g(z,\bar).
Given such a ''g'', we may produce a spin-weight ''s'' function by multiplying by a suitable power of the hermitian form
:P(z,\bar) = z\cdot\bar.
Specifically, ''f'' = ''P''−''s''''g'' is a spin-weight ''s'' function. The association of a spin-weighted function to an ordinary homogeneous function is an isomorphism.

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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