1. Progress in Mathematics;J. Moser,1982
2. M. J. Ablowitz and H. Segur,SIAM, Philadelphia, 1981.
3. Lecture Notes in Physics;P. P. Kulish,1982
4. E. L. Ince,Ordinary Differential Equations (Dover, New York, 1956).
5. H. T. Davis,Introduction to Nonlinear Differential and Integral Equations (Dover, New York, 1962).