1. Liénard, A., Étude des oscillations entreténues, Revue générale de l’électricité, 1928, vol. 23, pp. 901–912.
2. Liénard, A., Étude des oscillations entreténues, Revue générale de l’électricité, 1928, vol. 23, pp. 946–954.
3. Levinson, N. and Smith, O., A general equation for relaxation oscillations, Duke Math. J., 1942, vol. 9, no. 2, pp. 382–403.
4. Van der Pol, B., On relaxation-oscillations, The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, 1927, vol. 2, pp. 978–992.
5. Andronov, A.A., Vitt, A.A., and Khaikin, S.E., Teoriya kolebanii (Oscillation Theory), Moscow, 1959.