“…Then, by a paradox, they showed that a C-integrable equation must reduce to a linear equation or another C-integrable equation as the Eckhaus equation [4,22]. In the case of discrete equations it has been shown [26,1,16,18,17,19,10,11,12] that a similar situation is also true. One presents the equivalent of the Calogero-Eckhaus theorem stating that a necessary condition for a nonlinear dispersive partial difference equation to be S-integrable is that the lowest order multiple scale expansion on C (∞) functions give rise to integrable NLSE.…”