Separate plots of the curves U2 and T1. Separate plots of the curves U2 and T1 (see Eqs. (5) and (20)) for the case presented in Figs. 3 and 4. The minimum and maximum of the T1 curve coincide with the times for which the rate of growth is expected to have a minimum or a maximum, in correspondence with the inflection points in the y(t) curve.