Two-dimensional time-dependent Hamiltonian systems with an exact invariant

Abstract
We present a direct approach to investigate the existence of an exact invariant for two-dimensional Hamiltonians, in which the potential depends explicitly on time. The method is based on an expansion of the invariant in the velocities. The problem is solved completely for invariants linear and quadratic in the momenta. Our results contain as a particular case the results of Lewis and Leach on one-dimensional systems.