Limit laws for rational continued fractions and value distribution of quantum modular forms

Abstract

We study the limiting distributions of Birkhoff sums of a large class of cost functions (observables) evaluated along orbits, under the Gauss map, of rational numbers in $(0,1]$ ordered by denominators. We show convergence to a stable law in a general setting, by proving an estimate with power-saving error term for the associated characteristic function. This extends results of Baladi and Vall\'ee on Gaussian behaviour for costs of moderate growth. We apply our result to obtain the limiting distribution of values of several key examples of quantum modular forms. We show that central values of the Esterman function ($L$ function of the divisor function twisted by an additive character) tend to have a Gaussian distribution, with a large variance. We give a dynamical, "trace formula free" proof that central modular symbols associated with a holomorphic cusp form for $SL(2,{\bf Z})$ have a Gaussian distribution. We also recover a result of Vardi on the convergence of Dedekind sums to a Cauchy law, using dynamical methods.