Scaling of the rotation number for perturbations of rational rotations

TL;DR

This paper investigates how the rotation number of circle maps, which are perturbations of rational rotations, depends on parameters, establishing conditions for differentiability and linear scaling at critical points.

Contribution

It provides a criterion for the differentiability of the rotation number at critical parameters and derives an explicit formula for its derivative in terms of Fourier series.

Findings

Rotation number is differentiable at critical points under transversality.

Rotation number scales linearly at critical parameters.

Explicit derivative formula in terms of Fourier series.

Abstract

The parameter dependence of the rotation number in families of circle maps which are perturbations of rational rotations is described. We show that if, at a critical parameter value, the map is a (rigid) rotation $x\to x+\frac{p}{q}~({\rm mod}~1)$ with $p$ and $q$ coprime, then the rotation number is differentiable at that point provided a transversality condition holds, and hence that the rotation number scales linearly at this parameter. We provide an explicit and computable expression for the derivative in terms of the Fourier series of the map, and illustrate the results with the Arnold circle map and some modifications. Piecewise linear circle maps can also be treated using the same techniques.