Quantum Bi-Hamiltonian systems, alternative Hermitian structures and   Bi-Unitary transformations
  We discuss the dynamical quantum systems which turn out to be bi-unitary with respect to the same alternative Hermitian structures in a infinite-dimensional complex Hilbert space. We give a necessary and sufficient condition so that the Hermitian structures are in generic position. Finally the transformations of the bi-unitary group are explicitly obtained. 
G. Marmo, G. Scolarici, A. Simoni, F. Ventriglia