TheT, T ⁻¹-process, finitary codings and weak Bernoulli

Abstract

We give an elementary proof that the second coordinate (the scenery process) of theT, T ⁻¹-process associated to any mean zero i.i.d. random walk onZ ^d is not a finitary factor of an i.i.d. process. In particular, this yields an elementary proof that the basicT, T ⁻¹-process is not finitarily isomorphic to a Bernoulli shift (the stronger fact that it is not Bernoulli was proved by Kalikow). This also provides (using past work of den Hollander and the author) an elementary example, namely theT, T ⁻¹-process in 5 dimensions, of a process which is weak Bernoulli but not a finitary factor of an i.i.d. process. An example of such a process was given earlier by del Junco and Rahe. The above holds true for arbitrary stationary recurrent random walks as well. On the other hand, if the random walk is Bernoulli and transient, theT, T ⁻¹-process associated to it is also Bernoulli. Finally, we show that finitary factors of i.i.d. processes with finite expected coding volume satisfy certain notions of weak Bernoulli in higher dimensions which have been previously introduced and studied in the literature. In particular, this yields (using past work of van den Berg and the author) the fact that the Ising model is weak Bernoulli throughout the subcritical regime.