The distribution of the largest digit for parabolic Iterated Function Systems of the interval

We investigate the distribution of the largest digit for a wide class of infinite parabolic Iterated Function Systems (IFSs) of the unit interval. Due to the recurrence to parabolic (neutral) fixed points, the dimension analysis of these systems becomes more delicate than that of uniformly contracting IFSs. We show that the Hausdorff dimensions of level sets associated with the largest digits are constantly equal to the Hausdorff dimension of the limit set of the IFS. This result is an analogue of Wu and Xu’s theorem [Math. Proc. Cambridge Philos. Soc. 146 (2009), 207–212] on the regular continued fraction. Examples of application of our result include the backward (aka minus, or negative) continued fractions, even-integer continued fractions, and go beyond. Our main tool is a dimension theory for non-uniformly expanding Bernoulli interval maps with infinitely many branches.