Integrable Spatiotemporally Varying NLS, PT-Symmetric NLS, and DNLS Equations: Generalized Lax Pairs and Lie Algebras

TL;DR

This paper introduces two methods for deriving Lax-integrable nonlinear PDEs with spatiotemporally varying coefficients, including generalized NLS, PT-symmetric NLS, and DNLS equations, expanding the class of integrable systems.

Contribution

It develops a systematic approach using extended Lax pairs and generalized EW prolongation to derive new variable-coefficient integrable equations.

Findings

Derived generalized Lax pairs for variable-coefficient NLS, PT-symmetric NLS, and DNLS.

Showed that the techniques produce more general integrable systems than previous methods.

Demonstrated the effectiveness of the extended EW technique in systematizing the derivation process.

Abstract

This paper develops two approaches to Lax-integrbale systems with spatiotemporally varying coefficients. A technique based on extended Lax Pairs is first considered to derive variable-coefficient generalizations of various Lax-integrable NLPDE hierarchies recently introduced in the literature. As illustrative examples, we consider generalizations of the NLS and DNLS equations, as well as a PT-symmetric version of the NLS equation. It is demonstrated that the techniques yield Lax- or S-integrable NLPDEs with both time- AND space-dependent coefficients which are thus more general than almost all cases considered earlier via other methods such as the Painleve Test, Bell Polynomials, and various similarity methods. However, this technique, although operationally effective, has the significant disadvantage that, for any integrable system with spatiotemporally varying coefficients, one must 'guess' a generaliza- tion of the structure of the known Lax Pair for the corresponding system with constant coefficients. Motivated by the somewhat arbitrary nature of the above procedure, we therefore next attempt to systematize the derivation of Lax-integrable sytems with variable coefficients. We attempt to apply the Estabrook- Wahlquist (EW) prolongation technique, a relatively self-consistent procedure requiring little prior infomation. However, this immediately requires that the technique be significantly generalized or broadened in several different ways. The new and extended EW technique which results is illustrated by algorithmically deriving generalized Lax-integrable versions of NLS, PT-symmetric NLS, and DNLS equations.