1. J. Guckenheimer and P. Holmes, Nonlinear Oscillations, Dynamical Systems and Bifurcations of Vector Fields, Springer-Verlag, New York (1983).
2. Hao Bai-Lin, Chaos,World Scientific, Singapore, 1984
3. see for example P. Bak and J. Von Boehm, Phys. Rev., B21, 5297 (1980).
4. L. N. Bulaevskii and D. I. Khomskii, Soy. Phys. JETP, 47, 971 (1978).
5. M. Lowe and J. P. Gollub, Solitons and the commensurate-incommensurate transition in a convecting nematic fluid Phys. Rev., A31, 3893 (1985).