This solution arises from the spatial solution of the wave equation and diffusion equation. Here j ℓ (kr) and y ℓ (kr) are the spherical Bessel functions, and Y ℓ (θ, φ) are the spherical harmonics (Abramowitz and Stegun, 1964). Note that these forms are general solutions, and require boundary conditions to be specified to be used in any specific case. For infinite exterior domains, a radiation condition may also be required (Sommerfeld, 1949).