1. M. N. Gnevyshev, Izv. Glavn. Astron. Observ. Pulkovo 16, 36 (1938).
2. M. S. Eigenson, M. N. Gnevyshev, A. I. Ol’, and B. M. Rubashev, Solar Activity and Its Terrestrial Manifestations (Gos. Izdat. Tekhn.-Teor. Liter., Moscow, 1948) [in Russian].
3. H.W. Babcock and H. D. Babcock, Astrophys. J. 121, 349 (1955).
4. V. Bumba and R. Howard, Astrophys. J. 141, 1492 (1965).
5. V. Bumba and R. Howard, Astrophys. J. 141, 1502 (1965).