Zvyagintseva, Tatiana E. “On the Problem of Aizerman: Coefficient Conditions for an Existence of Four-Period Cycle in a Second-Order Discrete-Time System”. Vestnik of Saint Petersburg University. Mathematics. Mechanics. Astronomy, vol. 7, no. 1, May 2020, pp. 50-59, doi:10.21638/11701/spbu01.2020.105.