1. J.M. MANLEY and H.E. ROWE, Some General Properties of Nonlinear Elements I. -- General Energy Relations, Proc. Inst. Radio Engrs., vol. 44, pp.904–913, July 1956
2. P. PENFIELD, Jr., Frequency-Power Formulas, The Technology Press, Cambridge, Mass. and John Wiley and Sons, Inc., New York, N.Y., 1960
3. P.A. STURROCK, Action-Transfer and Frequency-Shift Relations in the Nonlinear Theory of Waves and Oscillations, Ann. Phys., vol. 9, pp. 422–434, March 1960
4. E. T. JAYNES, The Maser as a Parametric Amplifier, Quantum Electronics --A Symposium, Edited by C.H. Townes, Columbia University Press, New York, N.Y., 1960
5. M.T. WEISS, Quantum Derivation of Energy Relations Analogous to Those for Nonlinear Reactances, Proc. Inst. Radio Engrs., vol. 45, pp. 1012, 1013, July 1957