A.O. Korotkevich (1), A.C. Newell (2), and V.E. Zakharov (2,3,4,1)
  ((1) Landau Institute for Theoretical Physics RAS, (2) Department of
  Mathematics, University of Arizona, Tucson, (3) Lebedev Physical Institute
  RAS, (4) Waves and Solitons LLC, Phoenix)