Quasi-isospectral higher-order Hamiltonians via a reversed Lax pair construction

TL;DR

This paper introduces a new method for constructing higher-order Hamiltonians that are nearly isospectral by reversing the usual Lax pair interpretation, leading to novel integrable systems.

Contribution

It proposes a reversed Lax pair approach to generate quasi-isospectral Hamiltonians, expanding the toolkit for integrable systems and spectral analysis.

Findings

Constructed new higher-order Hamiltonians quasi-isospectral to each other.

Derived explicit examples from the KdV equation and extensions.

Presented infinite sequences of quasi-isospectral Hamiltonians, including shape-invariant operators.

Abstract

We present a novel approach for constructing quasi-isospectral higher-order Hamiltonians from time-independent Lax pairs by reversing the conventional interpretation of the Lax pair operators. Instead of treating the typically second-order $L$-operator as the Hamiltonian, we take the higher-order $M$-operator as the starting point and construct a sequence of quasi-isospectral operators via intertwining techniques. This procedure yields a variety of new higher-order Hamiltonians that are isospectral to each other, except for at least one state. We illustrate the approach with explicit examples derived from the KdV equation and its extensions, discussing the properties of the resulting operators based on rational, hyperbolic, and elliptic function solutions. In some cases, we present infinite sequences of quasi-isospectral Hamiltonians, which we generalise to shape-invariant differential operators capable of generating such sequences. Our framework provides a systematic mechanism for generating new integrable systems from known Lax pairs.