1. V. E. Zakharov andA. V. Mikhailov:Sov. Phys. JETP,47, 1017 (1979);V. E. Zakharov andS. V. Manakov:Soviet Science Review A, edited byV. Khalatnikov (New York, N. Y., 1979), p. 133.
2. V. E. Zakharov:The inverse scattering method, to be published.
3. N. I. Muskelishvili:Singular Integral Equations (Groningen, 1953).
4. Equation (4.3) reduces to the one derived byS. J. Orfanidis:Phys. Rev. D,18, 3828 (1978), by the position θ(n, t) = [u(n + 1,t) +u(n, t)]/2, θ(n, t) =u t (n, t)/2.