Abstract
We present a method to calculate numerically the action variables of a completely integrable Hamiltonian system with N degrees of freedom. It is a construction of the Liouville Arnol'd theorem for the existence of tori in phase space. By introducing a metric on phase space the problem of finding N independent irreducible paths on a given torus is turned into the problem of finding the lattice of zeros of an N-periodic function. This function is constructed using the flows of all constants of motion. Using the fact that neighbouring tori and their irreducible paths are related by some continuous deformation, a continuation method is constructed which allows a systematic scan of the actions. For N=2 we use a Poincare surface of section to define paths which cross neighbouring tori. Close to isolated periodic orbits the generators are either constructed explicitly or their asymptotic behaviour is given. As an example, the energy surface in the space of action variables of a Hamiltonian showing resonances is calculated.