Mixing for Free Semigroup Actions of Blaschke Products on the Circle

Abstract

We study mixing at a double exponential rate on analytic observables and ask how much of a map is remembered by its rate of mixing. For finite Blaschke products of the circle with a fixed point in the unit disk, and for the free semigroup actions they generate, we give a complete classification: the rate of mixing (no mixing, exponential, or double exponential) is determined by the multiplier of the generators at that fixed point, the invariant measure being the harmonic measure with a pole there. In the double exponential regime, the exponent equals $\log p$, where $p$ is the minimal local degree of the generators at the fixed point, and we show that this value is sharp. Consequently, the rate is not rigid: it is not stable under $C^1$-perturbations and does not imply $C^1$-conjugacy to affine models. Rigidity holds when the exponent is maximal for the degree: a map of degree $q$ whose exponent attains $\log q$ is Möbius conjugate to an affine model.