Tanaka–Ito α-continued fractions and matching

Abstract

Two closely related families of α-continued fractions were introduced in 1981: by Nakada on the one hand, by Tanaka and Ito on the other hand. The behavior of the entropy as a function of the parameter α has been studied extensively for Nakada’s family, and several of the results have been obtained exploiting an algebraic feature called matching. In this article we show that matching occurs also for Tanaka–Ito α-continued fractions, and that the parameter space is almost completely covered by matching intervals. Indeed, the set of parameters for which the matching condition does not hold, called the bifurcation set, is a zero measure set (even if it has full Hausdorff dimension). This property is also shared by Nakada’s α-continued fractions, and yet there also are some substantial differences: not only does the bifurcation set for Tanaka–Ito continued fractions contain infinitely many rational values, it also contains numbers with unbounded partial quotients.