By studying the weak closure of multidimensional off-diagonal self-joinings we provide a criterion for non-isomorphism of a flow with its inverse, hence the non-reversibility of a flow. This is applied to special flows over rigid automorphisms. In particular, we apply the criterion to special flows over irrational rotations, providing a large class of non-reversible flows, including some analytic reparametrizations of linear flows on T 2 , so called von Neumann's flows and some special flows with piecewise polynomial roof functions. A topological counterpart is also developed with the full solution of the problem of the topological self-similarity of continuous special flows over irrational rotations. This yields examples of continuous special flows over irrational rotations without topological self-similarities and having all non-zero real numbers as scales of measure-theoretic self-similarities.
ContentsClearly, J / ∈ SL 2 (R). However, if Γ satisfies Γ = J −1 ΓJ then J will also act on Γ\P SL 2 (R):and since J yields an order two map, we obtain that in this case the horocycle flow is reversible. It follows that I((h t ) t∈R ) = R * .Corollary 1.1. In the modular case Γ := P SL 2 (Z) ⊂ P SL 2 (R), the horocycle flow (h t ) t∈R is reversible.There are even cocompact lattices Γ which are not "compatible" with the matrix J. In this case a deep theory of Ratner [37] implies that in particular (h t ) t∈R is not measure-theoretically isomorphic to its inverse. 5 Notice that the same argument works for an arbitrary Koopman representation Ut = U Tt . In other words, an arbitrary Koopman representation is unitarily reversible.