1. Hayashi, C., Forced Oscillation in Non-linear Systems (Nippon Printing and Publishing Co., Osaka, Japan, 1953), pp.7-12.
2. Minnorsky, N., Introduction to Non-linear Mechanics (J. W. Edward, Ann Arbor, Mich. 1947), pp.9-19.
3. IV.—Researches into the Characteristic Numbers of the Mathieu Equation