V.D279 — Brightness Harmonic Decomposition

Brightness decomposition B(phi,theta) = Sum a_{nm} exp(i(n*phi+m*theta)) on T^2. Eigenvalues lambda_{nm} = n^2 + m^2*iota_tau^{-2}. Peter-Weyl theorem for U(1)×U(1), identical to V.D242.