Topological conjugacy of circle diffeomorphisms

The classical criterion for a circle diffeomorphism to be topologically conjugate to an irrational rigid rotation was given by Denjoy [1]. In [5] one of us gave a new criterion. There is an example satisfying Denjoy's bounded variation condition rather than the Zygmund condition of [5], and vice versa. This paper will give the third criterion which is implied by either of the above criteria.