Denjoy-Wolff points on the bidisk via models

Abstract

Let $F=(\phi, \psi):\mathbb{D}^2\to\mathbb{D}^2$ denote a holomorphic self-map of the bidisk without interior fixed points. It is well-known that, unlike the case with self-maps of the disk, the sequence of iterates $$\{F^n:=F\circ F\circ \cdots \circ F\}$$ needn't converge. The cluster set of $\{F^n\}$ was described in a classical 1954 paper of Herv\'{e}. Motivated by Herv\'{e}'s work and the Hilbert space perspective of Agler, McCarthy and Young on boundary regularity, we propose a new approach to boundary points of Denjoy-Wolff type for the coordinate maps $\phi, \psi.$ We establish several equivalent descriptions of our Denjoy-Wolff points, some of which only involve checking specific directional derivatives and are particularly convenient for applications. Using these tools, we are able to refine Herv\'{e}'s theorem and show that, under the extra assumption of $\phi$ and $\psi$ possessing Denjoy-Wolff points with certain regularity properties, one can draw much stronger conclusions regarding the behavior of $\{F^n\}.$