On the nonexistence of degenerate phase-shift discrete solitons in a dNLS nonlocal lattice
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We consider a one-dimensional discrete nonlinear Schr?dinger (dNLS) model featuring interactions beyond nearest neighbors. We are interested in the existence (or nonexistence) of phase-shift discrete solitons, which correspond to four-site vortex solutions in the standard two-dimensional dNLS model (square lattice), of which this is a simpler variant. Due to the specific choice of lengths of the inter-site interactions, the vortex configurations considered present a degeneracy which causes the standard continuation techniques to be non-applicable. In the present one-dimensional case, the existence of a conserved quantity for the soliton profile (the so-called density current), together with a perturbative construction, leads to the nonexistence of any phase-shift discrete soliton which is at least C2 with respect to the small coupling ? in the limit of vanishing ?. If we assume the solution to be only C0 in the same limit of ? nonexistence is instead proved by studying the bifurcation equation of a Lyapunov�Schmidt reduction, expanded to suitably high orders. Specifically, we produce a nonexistence criterion whose efficiency we reveal in the cases of partial and full degeneracy of approximate solutions obtained via a leading order expansion. 2017 Elsevier B.V.
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