Zonal spherical harmonics explained

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$$Z^(\backslash theta,\backslash phi)\; =\; P\_\backslash ell(\backslash cos\backslash theta)$$where is a Legendre polynomial of degree . The general zonal spherical harmonic of degree ℓ is denoted by

Z ^{(\ell)} _{x} ( y ) , 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 ^{(\ell)} (\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 ^{(\ell)} _{x} to be the dual representation of the linear functional$$P\backslash mapsto\; P(\backslash mathbf)$$in the finite-dimensional Hilbert space H_ℓ of spherical harmonics of degree ℓ. In other words, the following reproducing property holds:$$Y(\backslash mathbf)\; =\; \backslash int\_\; Z^\_(\backslash mathbf)Y(\backslash mathbf)\backslash ,d\backslash Omega(y)$$for all . 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 Rⁿ: for x and y unit vectors,$$\backslash frac\backslash frac$$

^n

= \sum_^\infty r^k Z^_(\mathbf),where \omega _{n-1} is the surface area of the (n-1)-dimensional sphere. They are also related to the Newton kernel via$$\backslash frac$$

^

= \sum_^\infty c_ \frac

^k

^

Z_^(\mathbf/|\mathbf|)where and the constants are given by$$c\_\; =\; \backslash frac\backslash frac.$$

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, then$$Z^\_(\backslash mathbf)\; =\; \backslash fracC\_\backslash ell^(\backslash mathbf\backslash cdot\backslash mathbf)$$where are the constants above and

C ^{(\alpha)} _{\ell} is the ultraspherical polynomial of degree ℓ.

Properties

• The zonal spherical harmonics are rotationally invariant, meaning that $$Z^\_(R\backslash mathbf)\; =\; Z^\_(\backslash mathbf)$$ for every orthogonal transformation R. Conversely, any function on that is a spherical harmonic in y for each fixed x, and that satisfies this invariance property, is a constant multiple of the degree zonal harmonic.
• If Y₁, ..., Y_d is an orthonormal basis of, then $$Z^\_(\backslash mathbf)\; =\; \backslash sum\_^d\; Y\_k(\backslash mathbf)\backslash overline.$$
• Evaluating at gives $$Z^\_(\backslash mathbf)\; =\; \backslash omega\_^\; \backslash dim\; \backslash mathbf\_\backslash ell.$$

References

• .