A priori bounds and degeneration of Herman rings with bounded type rotation number

Abstract: By adapting the near-degenerate regime designed by Kahn, Lyubich, and D. Dudko, we prove that the boundaries of Herman rings with bounded type rotation number and of the simplest configuration are quasicircles with dilatation depending only on the degree and the rotation number. As a consequence, we show that these Herman rings always degenerate to a Herman curve, i.e. an invariant Jordan curve that is not contained in the closure of a rotation domain and on which the map is conjugate to a rigid rotation. This process enables us to construct the first general examples of rational maps having Herman curves of bounded type with arbitrary degree and combinatorics. In particular, they do not come from Blaschke products. We also demonstrate the existence of Renormalization Theory for Herman curves by constructing rescaled limits of the first return maps in the unicritical case.