Darboux' theorem for Hamiltonian differential operators

It is proved that any one-dimensional, first order Hamiltonian differential operator can be put into constant coefficient form by a suitable change of variables. Consequently, there exist canonical variables for any such Hamiltonian operator. In the course of the proof, a complete characterization of all first order Hamiltonian differential operators, as well as the general formula for the behavior of a Hamiltonian operator under a change of variables involving both the independent and the dependent variables are found.