R.R, Nigmatullin, and Toboev V.A. “Non-Orthogonal Amplitude-Frequency Analysis of the Smoothed Signals(NAFASS): Dynamics and the Fine Structure of the Sunspots”. Journal of Applied Nonlinear Dynamics, vol. 4, no. 1, Aug. 2026, pp. 67-80, https://doi.org/10.5890/JAND.2015.03.006.