1. Moiseyev, N. N. 1958 Prikl. Mat. Mekh. 22,612.
2. Ursell, F. 1952 Proc. Roy. Soc. A,214,79.
3. Miles, J. W. 1967 Proc. Roy. Soc. A,297,459.
4. Davis, H. T. 1962 Introduction to Nonlinear Differential and Integral Equations .Dover.
5. Cole, J. D. 1968 Perturbation Methods in Applied Mathematics .Waltham, Mass.:Blaisdell.