“…Equation (1.2) is related to the self-dual Yang-Mills (SDYM) equation, and has been studied by several researchers from various viewpoints: Lax pairs [Bo2,Sc,St1], Hirota bilinear method [SOM,St2], twistor approach [St2], Painlevé analysis [JB], and so on. In the case X = Y , this equation is reduced to iu T + u XX + 2|u| 2 u = 0, (1.3) which is the celebrated Nonlinear Schrödinger (NLS) equation.…”