### Introduction

Solid harmonics are solutions of the Laplace operator in spherical coordinates:

$$\nabla^2\phi =\frac{1}{r^2}\left[\frac{\partial}{\partial r}\left(r^2\frac{\partial\phi}{\partial r}\right)+\frac{1}{\sin\theta} \frac{\partial}{\partial\theta}\left(\sin\theta\frac{\partial\phi}{\partial\theta}\right)+ \frac{1}{\sin^2\theta}\frac{\partial^2\phi}{\partial\lambda^2}\right] = 0.$$

This relationship holds because spherical harmonics are eigenfunctions of the surface Laplace (Laplace-Beltrami) operator, i.e.,

$$ \frac{1}{\sin\theta}\frac{\partial}{\partial\theta} \left(\sin\theta\frac{\partial Y^m_l}{\partial\theta}\right)+ \frac{1}{\sin^2\theta} \frac{\partial^2 Y^m_l}{\partial\lambda^2} = -l(l+1)Y^m_l,$$

where $Y^m_l$ stands for the normalized spherical harmonic of degree $l$ and order $m$. Substitution of $\phi=F(r)Y^m_l$ into the Laplace equation gives

$$\frac{\partial}{\partial r}\left(r^2\frac{\partial FY^m_l}{\partial r}\right)=l(l+1)FY^m_l.$$

Therefore, general solutions to this equation are of the form $F(r) = Ar^l$ or $F(r)=Br^{-l-1}$. The particular solutions of the Laplace equations are regular solid harmonics, i.e.,

$$R^m_l(r,\lambda,\theta) = a_{lm}r^lY^m_l(\lambda,\theta),$$

which vanish at the origin, and irregular solid harmonics, i.e.,

$$I^m_l(r,\lambda,\theta) = a_{lm}\frac{Y^m_l(\lambda,\theta)}{r^{l+1}}.$$

In this example, we will use solid harmonics to mean the regular solid harmonics $R^m_l$ since the irregular solid harmonic possess a singularity at the origin. The solid harmonics are normalized so that their 2-norm is equal to $1$:

$$\int_B R^m_l R^{m}_ldV=1.$$

Thus, we have

$$a_{lm}^2\int_0^1r^{2l}r^2dr\int_{\partial B}Y^m_lY^{m}_ldS=1,$$

so that $a_{lm} = \sqrt{2l+3}$.

### Solid harmonics in Ballfun

Solid harmonics can be constructed in Ballfun by calling the command `solharm`

. This creates a solid harmonic of a given degree and order. For example, $R^2_4$ can be constructed and plotted as follows:

R42 = ballfun.solharm(4, 2); plot( R42 ), axis off

We can verify that this function is an eigenfunction of the Laplace operator with zero eigenvalue as follows:

norm( laplacian( R42 ) )

ans = 1.888485572980376e-14

The solid harmonics are also orthonormal on the ball with respect to the standard $L^2$ inner-product. This can be verified with the `sum3`

command:

R40 = ballfun.solharm(4, 0); sum3( R42*R42 ) sum3( R40*R40 ) sum3( R42*R40 )

ans = 0.999999999999999 ans = 1.000000000000000 ans = 0

Here is a plot of the solid harmonics $R^m_l$, with $l=0,...,4$ and $0\leq m\leq l$.

N = 3; for l = 0:N for m = 0:l R = ballfun.solharm(l,m); subplot(N+1,N+1,l*(N+1)+m+1), plot(R) axis off end end

### Computing solid harmonic coefficients

A fast and stable algorithm for computing the solid harmonics is implemented in Ballfun. The computation of the solid harmonics $R^m_l$ of degree $l$ and order $m$ requires $\mathcal{O}(l\log l)$ operations. The solid harmonics $R^m_l$ can be expressed as

$$R^m_l(r,\lambda,\theta) = \sqrt{2l+3}r^lY^m_l(\lambda,\theta),$$

where $Y^m_l$ stands for the spherical harmonic of degree $l$ and order $m$.

The main issue in the computation of $R^m_l$ is to find the Fourier coefficients of the associated Legendre polynomial of degree $l$ and order $m$, $P^m_l$. Thus, the algorithm used in `solharm`

to compute the coefficients of this polynomial is the Modified Forward Column (MFC) method described in [3].

The most popular recursive algorithm [1] (Forward Column method) that computes non-sectoral ($l>m$) $P^m_l$ from previously computed $P^m_{l-1}$ and $P^m_{l-2}$ is given by

$$P^m_l(\theta) = a_{lm}\cos(\theta)P^m_{l-1}(\theta)+b_{lm}P^m_{l-1},\quad \forall l>m,$$

where

$$P^m_l(\theta) = a_{lm}\cos(\theta)P^m_{l-1}(\theta)+b_{lm}P^m_{l-1},\quad \forall l>m,$$

and

$$b_{lm} = \sqrt{\frac{(2l+1)(l+m+1)(n-m-1)}{(l-m)(l+m)(2l-3)}}.$$

The sectoral ($l=m$) $P^m_l$ can be computed using the initial values $P^0_0(\theta)=1$ and $P^1_1(\theta)=\sqrt(3)\sin(\theta)$ and the recursion [1]

$$P^m_m(\theta)=\sin(\theta)\sqrt{\frac{2m+1}{2m}}P^{m-1}_{m-1},\quad \forall m>1.$$

Using these recursions, $P^m_l(\theta)$ is evaluated at $2l+1$ points and the coefficients are then recovered by an FFT. The recursive algorithm is unstable and will overflow for large degrees $l>1900$ [2] because of the factors $\sin(\theta)^m$ in $P^m_l$.

The idea of the MFC method is to compute $P^m_l(\theta)/\sin(\theta)^m$ in the recursion and then multiply by $\sin(\theta)^m$ at the end before the FFT. The recursion to compute the sectoral values of $P^m_l/\sin(\theta)^m$ remains unchanged:

$$\frac{P^m_l(\theta)}{\sin(\theta)^m} = a_{lm}\cos(\theta) \frac{P^m_{l-1}(\theta)}{\sin(\theta)^m}+b_{lm} \frac{P^m_{l-1}}{\sin(\theta)^m},\quad \forall l>m.$$

Finally, the sectoral values of $P^m_m(\theta)/\sin(\theta)^m$ are computed using the initial values $P^0_0(\theta)/\sin(\theta)^0 = 1$ and $P^1_1(\theta)/\sin(\theta) = \sqrt{3}$ and the relationship

$$P^m_m(\theta)=\sqrt{\frac{2m+1}{2m}}P^{m-1}_{m-1},\quad \forall m>1.$$

The computation of the solid harmonics is very fast in Ballfun and a solid harmonics of degree $150$ can be computed in a few tenths of a second:

tic ballfun.solharm(150, 50); toc

Elapsed time is 0.369532 seconds.

### References

[1] O. L. Colombo, Numerical methods for harmonic analysis on the sphere, report DGS-310, Department of Geodetic Science and Surveying, Ohio State University, 1981.

[2] D. M. Gleason, Partial sums of Legendre series via Clenshaw summation, *Manuscr. Geod.*, 10 (1985), pp. 115-130.

[3] S. A. Holmes and W. E. Featherstone, A unified approach to the Clenshaw summation and the recursive computation of very high degree and order normalised associated Legendre functions, *Journal of Geodesy*, 76 (2002), pp 279-299.