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

• URL: http://arxiv.org/abs/2601.20752v1
• Date: Wed, 28 Jan 2026 16:33:29 GMT
• Title: Spectrum-generating algebra and intertwiners of the resonant Pais-Uhlenbeck oscillator
• Authors: Andreas Fring, Ian Marquette, Takano Taira,
• Abstract summary: We study the quantum Pais-Uhlenbeck oscillator at the resonant (equal-frequency) point.<n>We show that quantising a classically equivalent Hamiltonian yields a radically different quantum theory.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• 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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