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Yang C N 2009 Chin. Phys. Lett. 26 120504. We
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[5] You Y Z 2009 Chin. Phys. Lett. (submitted) Notice
You's $f$ and $\alpha$ are related to $\zeta$ and $\beta$ of the
present paper by $f=2\zeta /\beta^2$ and $\alpha =1/\beta$, where
the factor 2 comes from a comparison of You's Hamiltonian with ours.