Abstract
We first exhibit two compatible Poisson structures on the cotangent bundle of the unitary group \(\mathrm{U}(n)\) in such a way that the invariant functions of the \({\mathfrak {u}}(n)^*\)-valued momenta generate a bi-Hamiltonian hierarchy. One of the Poisson structures is the canonical one and the other one arises from embedding the Heisenberg double of the Poisson–Lie group \(\mathrm{U}(n)\) into \(T^*\mathrm{U}(n)\), and subsequently extending the embedded Poisson structure to the full cotangent bundle. We then apply Poisson reduction to the bi-Hamiltonian hierarchy on \(T^* \mathrm{U}(n)\) using the conjugation action of \(\mathrm{U}(n)\), for which the ring of invariant functions is closed under both Poisson brackets. We demonstrate that the reduced hierarchy belongs to the overlap of well-known trigonometric spin Sutherland and spin Ruijsenaars–Schneider-type integrable many-body models, which receive a bi-Hamiltonian interpretation via our treatment.
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Notes
If not specified otherwise, our spaces of \(C^\infty \)-functions always denote spaces of real functions.
Incidentally, one can also work out a direct proof of the closure of \(C^\infty ({\mathfrak {M}})^{\mathrm{U}(n)}\) under \(\{\ ,\ \}_2\) (3.5).
We here use some well-known results about free proper actions; see, e.g. paragraph 6.5 in [36].
A complicated explicit formula for \(b_+(Q,\lambda )\) can be obtained along the lines of Section 5.2 in [13].
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Acknowledgements
I wish to thank I. Marshall for generous help with some calculations. I am also grateful to J. Balog, T.F. Görbe and M. Fairon for useful comments on the manuscript. This research was performed in the framework of the Project GINOP-2.3.2-15-2016-00036 co-financed by the European Regional Development Fund and the budget of Hungary.
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Fehér, L. Reduction of a bi-Hamiltonian hierarchy on \(T^*\mathrm{U}(n)\) to spin Ruijsenaars–Sutherland models. Lett Math Phys 110, 1057–1079 (2020). https://doi.org/10.1007/s11005-019-01252-1
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DOI: https://doi.org/10.1007/s11005-019-01252-1