Percentage difference between Δa_1, 0 computed for the analytical description of the EDGES absorption profile using Eq. (10) with all the terms and taking only the contribution from the first-order derivative, in the ideal case without errors. We note that the expected divergence at a frequency of around ≃73.5 MHz disappears only due to frequency sampling. Solid (dots) lines refer to positive (negative) values. See text.