Any linear combination of real spherical harmonics
for fixed whose sum is not premultiplied by a factor (Whittaker and Watson 1990, p. 392).
Any linear combination of real spherical harmonics
for fixed whose sum is not premultiplied by a factor (Whittaker and Watson 1990, p. 392).
Weisstein, Eric W. "Surface Harmonic." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/SurfaceHarmonic.html