Smooth integrability of diffeomorphisms
Motivated by the notion of integrability introduced by Bogoyavlenskij for vector fields, we propose a definition of smooth integrability for general diffeomorphisms. In brief, we say that a diffeomorphism is integrable if it commutes with the flows of {\color{black}commutative} vector fields and shares their first integrals. We establish a Liouville-Arnold type theorem and prove that on connected invariant level sets of the first integrals, a smoothly integrable diffeomorphism is conjugate to a skew translation on a toroidal cylinder, and in particular, when the level sets are compact, the induced dynamics is quasiperiodic, as in the classical Hamiltonian case. We further show that linear diffeomorphisms on real or complex Euclidean spaces are integrable in our sense, as is the case for linear vector fields in the sense of Bogoyavlenskij, and symplectic diffeomorphisms that are integrable in their standard Liouville-type definition are integrable in our framework. We also modify the cotangent lift construction, which is useful in the treatment of integrability of vector fields, for diffeomorphisms, and show that integrability is preserved under this correspondence. Several examples are provided to illustrate the scope of the theory.