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Zonal spherical harmonics : ウィキペディア英語版
Zonal spherical harmonics
In the mathematical study of rotational symmetry, the zonal spherical harmonics are special spherical harmonics that are invariant under the rotation through a particular fixed axis. The zonal spherical functions are a broad extension of the notion of zonal spherical harmonics to allow for a more general symmetry group.
On the two-dimensional sphere, the unique zonal spherical harmonic of degree ℓ invariant under rotations fixing the north pole is represented in spherical coordinates by
:Z^(\theta,\phi) = P_\ell(\cos\theta)
where ''P'' is a Legendre polynomial of degree ℓ. The general zonal spherical harmonic of degree ℓ is denoted by Z^_), where x is a point on the sphere representing the fixed axis, and y is the variable of the function. This can be obtained by rotation of the basic zonal harmonic Z^(\theta,\phi).
In ''n''-dimensional Euclidean space, zonal spherical harmonics are defined as follows. Let x be a point on the (''n''−1)-sphere. Define Z^_)
in the finite-dimensional Hilbert space H of spherical harmonics of degree ℓ. In other words, the following reproducing property holds:
:Y(\mathbf) = \int__)Y(\mathbf)\,d\Omega(y)
for all ''Y'' ∈ H. The integral is taken with respect to the invariant probability measure.
==Relationship with harmonic potentials==
The zonal harmonics appear naturally as coefficients of the Poisson kernel for the unit ball in R''n'': for x and y unit vectors,
:\frac|^n} = \sum_^\infty r^k Z^_),
where \omega_ is the surface area of the (n-1)-dimensional sphere. They are also related to the Newton kernel via
:\frac|^} = \sum_^\infty c_ \frac}Z_|}^(\mathbf/|\mathbf|)
where x,y ∈ R''n'' and the constants ''c''''n'',''k'' are given by
:c_ = \frac.
The coefficients of the Taylor series of the Newton kernel (with suitable normalization) are precisely the ultraspherical polynomials. Thus, the zonal spherical harmonics can be expressed as follows. If α = (''n''−2)/2, then
:Z^_) = \frac(\mathbf\cdot\mathbf)
where ''c''''n'',ℓ are the constants above and C_\ell^ is the ultraspherical polynomial of degree ℓ.

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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