The n:m resonance dual pair
  In this paper we build dual pairs of Poisson maps \[ \RR\stackrel{R_\pm}{\longleftarrow}(D,\om_\pm) \stackrel{\Pi_\pm}{\longrightarrow} B \] associated to $n:m$ resonance, as well as to $n:-m$ resonance. Except for the above mentioned cases $1:\pm1$, these are not pairs of momentum maps. Here $D$ is an open subset of $\CC^2$ with the above mentioned symplectic forms $\om_\pm$, and $B$ an open subset of $\RR^3$. The Poisson structure on $B$, which depends on the natural numbers $n$ and $m$, is not Lie-Poisson. Instead, its symplectic leaves are the Kummer shapes: bounded surfaces for $n:m$ resonance, and unbounded surfaces for $n:-m$ resonance 
Darryl D. Holm and Cornelia Vizman
math.DS nlin.CD