The Spherical Harmonic Spectrum of a Function with Algebraic Singularities

摘要

The asymptotic behaviour of the spectral coefficients of a function provides a useful diagnostic of its smoothness. On a spherical surface, we consider the coefficients a_1 ~m of fully normalised spherical harmonics of a function that is smooth except either at a point or on a line of colatitude, at which it has an algebraic singularity taking the form θ~p or {pipe}θ-θ_0{pipe}p respectively, where θ is the co-latitude and p>-1. It is proven that each type of singularity has a signature on the rotationally invariant energy spectrum, E(l) = √Σ_m(a_1 ~m)~2 where l and m are the spherical harmonic degree and order, of l~(-(p+3/2)) or l~(-(p+1)) respectively. This result is extended to any collection of finitely many point or (possibly intersecting) line singularities of arbitrary orientation: in such a case, it is shown that the overall behaviour of E(l) is controlled by the gravest singularity. Several numerical examples are presented to illustrate the results. We discuss the generalisation of singularities on lines of colatitude to those on any closed curve on a spherical surface.