On restoring forces which admit forcing terms of non-critical amplitude
- K. O. Friedrichs and J. J. Stoker, Forced vibrations of systems with non-linear restoring force, Quarterly of Applied Mathematics vol. 1 (1943) pp. 97-115. MR 8292
- S. Timoshenko, Vibration problems in engineering, Van Nostrand, New York, 1937, p. 120.