1. H. Airault, H. McKean, and J. Moser, Courant Institute Preprint (1976).
2. L. A. Bordag and V. B. Matveev, Leipzig Univ. Preprint (1977).
3. D. V. Choodnovsky and G. V. Choodnovsky, Marconi Inst. Preprint (1977).
4. L. A. Bordag, A. R. Its, V. B. Matveev, S. V. Manakov, and V. E. Zakharov, Leipzig Univ. Preprint (1977).
5. B. A. Dubrovin and S. P. Novikov, Zh. Eksp. Teor. Fiz.,67, No. 12, 2131–2143 (1974).