Hirota method for oblique solitons in two-dimensional supersonic   nonlinear Schroedinger flow
  In a previous work[1] exact stable oblique soliton solutions were revealed in two dimensional nonlinear Schroedinger flow. In this work we show that single soliton solution can be expressed within the Hirota bilinear formalism. An attempt to build two-soliton solutions shows that the system is "close" to integrability provided that the angle between the solitons is small and/or we are in the hypersonic limit. 
E. G. Khamis and A. Gammal
nlin.PS cond-mat.quant-gas