spherical vector harmonic | vector harmonic

The spherical harmonics can be generalized to vector spherical harmonics by looking for a scalar function ψ and a constant vector c such that M | congruent | del x(c ψ) | = | ψ( del xc) + ( del ψ)xc | = | ( del ψ)xc | = | -cx del ψ, so del ·M = 0.