1. Berghmans, D., van der Linden, R.A.M., and Vanlommel, P., et al., History of the sunspot index: 25 years SIDC, Beitr-. Gesch. Geophys. Kosm. Phys., 2006, vol. 7, no. 1, p. 288.
2. Bogdan, T.J., Gilman, P.A., and Lerche, I.R.H., Distribution of sunspot umbral areas: 1917–1982, Astrophys. J., 1988, vol. 327, p. 451.
3. Bray, R.J. and Loughead, R.E., Sunspots, New York: John Wiley and Sons, 1964.
4. Chernan, C.M., The Handbook of Solar Flare Monitoring and Propagation Forecasting, Summit, Pa.: Tab Books, 1978.
5. Clette, F., Svalgaard, L., Vaquero, J.M., and Cliver, E.W., Revisiting the sunspot number: A 400-year perspective on the solar cycle, Space Sci. Rev., 2014, vol. 186, p. 35.