CARTWRIGHT, M. L. Forced oscillations in nearly sinusoidal systems. J. Inst. Elec. Eng. 1948, vol.95, 88-94
CARTWRIGHT, M. L. Forced oscillations in nonlinear systems. Contributions to the Theory of Nonlinera Oscillations, vol.I, Ann. of Math. Ser., no.20, Princeton U. P. 1950
FLAHERTY, J. E. Frequency entrainment of a forced van der Pol oscillator. Studies in Applied Mathematics. 1978, vol.58, 5-15