Quad-equations and auto-Bäcklund transformations of NLS-type systems

Abstract: Treating an integrable quad-equation along with its two generalised symmetries as a compatible system allows one to construct an auto-B\"acklund transformation for solutions of the related NLS-type system. A fixed periodic reduction of the quad-equation yields a quasi-periodic reduction of its generalised symmetries that turn them into differential constraints compatible with the NLS-type system.