1. Introduction to Non-linear Mechanics;Kryloff,1943
2. Introduction to Non-linear Mechanics;Minorsky,1947
3. Forced Oscillations in Non-linear System;Cartwright,1950
4. Non-linear Aspects of Sampled-Data Control Systems;Kalman,1956
5. The Calculus of Finite Differences;Milne-Thomson,1951