If the functions "f" and "g" have a zero mean (i.e., the spectral coefficients "f"00 and "g"00 are zero), then "S""ff"(ℓ) and "S""fg"(ℓ) represent the contributions to the function's variance and covariance for degree ℓ, respectively.
If the functions "f" and "g" have a zero mean (i.e., the spectral coefficients "f"00 and "g"00 are zero), then "Sff"("l") and "Sfg"("l") represent the contributions to the function’s variance and covariance for degree formula_23, respectively.