“…Accordingly, the latter is here seen to be gauge equivalent to the canonical integrable cubic NLS equation in . Moreover, if, in addition, q c = 0 then (25) reduces to the Kundu-Eckhaus equation, linked to a linear Schrödinger equation in via gauge transformation (20). Here, we shall be concerned with the general case with q c = 0, q q = 0, s = 0 and alignment of (24) is made with the nonlinear capillarity model (19).…”