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arXiv · 2601.20752

Spectrum-generating algebra and intertwiners of the resonant Pais-Uhlenbeck oscillator

Andreas Fring·Ian Marquette·Takano Taira
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Abstract

We study the quantum Pais-Uhlenbeck oscillator at the resonant (equal-frequency) point, where the dynamics becomes non-diagonalisable and the conventional Fock-space construction collapses. At the classical level, the degenerate system admits more than one Hamiltonian formulation generating the same equations of motion, leading to a nontrivial quantisation ambiguity. Working first in the ghostly two-dimensional Hamiltonian formulation, we construct differential intertwiners that generate a spectrum-generating algebra acting on the generalised eigenspaces of the Hamiltonian. This algebra organises the generalised eigenvectors into finite Jordan chains and closes into a hidden $su(2)$ Lie algebra that exists only at resonance. We then show that quantising a classically equivalent Hamiltonian yields a radically different quantum theory, with a fully diagonalisable spectrum and genuine degeneracies. Our results demonstrate that the resonant Pais-Uhlenbeck oscillator provides a concrete example in which classically equivalent Hamiltonians define inequivalent quantum theories.

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Andreas Fring, Ian Marquette, Takano Taira. 2026-01-28. Spectrum-generating algebra and intertwiners of the resonant Pais-Uhlenbeck oscillator. https://doi.org/10.1088/1751-8121/ae8bf0

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