the existence of shift for periodically correlated processes and its boundedness are investigated. spectral criteria for these non-stationary processes to have such shifts are obtained.

Let M be a smooth manifold and Φ be a smooth flow on M . We study the maps M → M generated by smooth shifts along trajectories of Φ, i.e the maps of a form h(x) = Φ(x, σ(x)) where σ is some smooth function on M . We will classify smooth shifts which induce diffeomorphisms ofM and prove contractibility of the set of such diffeomorphisms for large class of flows. These results will be applied to ...