Wigner's Problem and Alternative Commutation Relations for Quantum   Mechanics
  It is shown, that for quantum systems the vectorfield associated with the equations of motion may admit alternative Hamiltonian descriptions, both in the Schr\"odinger and Heisenberg pictures. We illustrate these ambiguities in terms of simple examples. 
V. I. Man'ko, G. Marmo, E. C. G. Sudarshan, and F. Zaccaria