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<ref name="Cartwright_Littlewood_On_non-linear_differential_equations_of_the_second_order"> {{cite journal | author=Cartwright M L | author2=Littlewood J E | title=On non-linear differential equations of the second order: I. The equation \ddot{y} - k(1 - y^2)\dot{y} + y = b\lambda k \cos(\lambda t + a), k large | journal=Journal of the London Mathematical Society | year=1945 | volume=20 | issue=3 | pages=180-189 | doi=10.1112/jlms/s1-20.3.180 }} </ref>

<ref name="Cartwright_Littlewood_On_non-linear_differential_equations_of_the_second_order"> {{cite journal | author=Cartwright M L | author2=Littlewood J E | title=On non-linear differential equations of the second order: I. The equation \ddot{y} - k(1 - y^2)\dot{y} + y = b\lambda k \cos(\lambda t + a), k large | journal=Journal of the London Mathematical Society | year=1945 | volume=20 | issue=3 | pages=180-189 | doi=10.1112/jlms/s1-20.3.180 }} </ref>

<ref name="Hayashi_Shibayama_Ueda_Quasi-periodic_Oscillations_in_Self-oscillatory_Systems_with_External_Force"> {{cite journal | author=Hayashi C | author2=Shibayama H | author3=Ueda Y | title=Quasi-periodic Oscillations in Self-oscillatory Systems with External Force | journal=IECE Technical Report, Nonlinear Theory | year=1961 | date=16 December 1961 }} </ref>

<ref name="Hayashi_Shibayama_Ueda_Quasi-periodic_Oscillations_in_Self-oscillatory_Systems_with_External_Force"> {{cite journal | author=Hayashi C | author2=Shibayama H | author3=Ueda Y | title=Quasi-periodic Oscillations in Self-oscillatory Systems with External Force | journal=IECE Technical Report, Nonlinear Theory | year=1961 | date=16 December 1961 }} </ref>