“…A frequently used choice of basis functions for the spectral method are the spherical harmonics [23]. They are eigenfunctions of the spherical Laplacian, which makes them a natural choice for problems such as solving differential equations [39], spherical deconvolution [16], the approximation of measures [11], or modeling the earth's upper mantle [15,38] and gravitational field [13]. While the naive computation of spherical harmonics expansions is very memory and time consuming [39], there are methods allowing for a faster computation, known as fast spherical Fourier transforms, see, e.g., [10,19,24,37] and [28, sec.…”