Δa_ℓ, 0 for the considered non-equilibrium model (ν₀ ≃ 0.35 GHz and α ≃ 3.36), assuming a value of β multiplied by a factor 100; ℓ ranges from 1 to ℓ_max = 6 in the case of the solutions described in Sect. 3 and up to 8 in the case of numerical integration (with the routine D01AJF) based on Eq. (5). Solid lines (or dots) correspond to positive (or negative) values. Green and red lines are essentially superimposed up to ℓ = 6. Their difference, multiplied by a factor F to have values compatible with the adopted range, is displayed by the blue lines. Yellow lines refer to the nominal integration error quoted by the routine D01AJF, again multiplied by the factor F. See also the legend and the text.