12 september 2005, kl 13.15-15.15, Uppsala, rum 2244, DUBBELSEMINARIUM

Anders Öberg (Uppsala): Uniqueness of $g$-measures

Abstract. In this talk I will survey some developments in ergodic theory which I have been involved in. The talk will be easily accessible to everyone.
    In 1972 (Invent. Math.) Michael Keane introduced $g$-measures into ergodic theory. These are invariant measures for a map $T$ with a probability function, a $g$-function, which is a transition probability for the local inverses of $T$. Finding regularity conditions on the $g$-functions, to ensure uniqueness of $g$-measures, is a central problem. Keane's results were extended by Peter Walters in the 1970s and Walters used summability of variations of the $g$-functions to ensure uniqueness of $g$-measures. In the eighties more complicated conditions were used by Berbee, but these are still in the realm of $\ell^1$. Such conditions had also been available in the Markov chain literature, already in the 1950s. But there had also been mistakes: it had been claimed (in similar contexts) that uniqueness holds for transition probabilities which are only continuous, but strictly positive.
    In 1993 (Israel J. Math.) Bramson and Kalikow provided a first counterexample to uniqueness when the $g$-functions are only continuous (but strictly positive). In 2003 (Math. Res. Lett.) Anders Johansson and I proved that square summability of variations of the $g$-functions is enough for uniqueness of $g$-measures, and now Berger, Hoffman and Sidoravicius (preprint 2004) have proved that this condition is also necessary in a certain sense: For any $\epsilon>0$ there exists $g$-functions whose variations are in $\ell^{2+\epsilon}$ and for which there are two distinct $g$-measures.
    Recently, Johansson, \"Oberg and Pollicott proved that the condition of square summability of variations of the $g$-functions can be generalized in a natural way to ensure uniqueness of $g$-measures for countable state shifts.