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An implicit formula for the spectrum of the two-photon Rabi model was presented by Trav {e} {e}{}nec in Phys. Rev. A 85, 043805 (2012), in analogy to the method of Braak [Phys. Rev. Lett. 107, 100401 (2011)]. The spectrum is given by common zeros of four functions Gc(z,E) taken with a fixed value of z∈C. We point out that all the Gc(z,E) functions defined by Trav {e} {e}{}nec are identically zero, thus the spectrum is not in fact given by their roots.
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Maciejewski et al. (2015) studied this question.
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