# Formal Power Series Solutions of Schröder's Equation

## Document Type

Article

## Publication Date

9-1-2007

## Department

Mathematics & Statistics

## Abstract

In 1884, Königs showed that when φ(z) is an analytic self-map of the unit disk fixing the origin, with 0 less than | φ′ (0)| less than 1, then Schröder's functional equation, σ, o φ(z) = φ′ (0)σ(z), can be solved for a unique analytic function σ(z) in the disk with σ′(0) = 1. In 2003, Cowen and MacCluer considered an analogue of Schröder's equation in the unit ball of ℂ for N greater than 1. Under some natural hypotheses, they gave necessary and sufficient conditions for the existence of an analytic solution σ satisfying σ′(0) = I when φ′(0) is diagonalizable. In this paper, the problem when φ′(0) is not diagonalizable is considered. Both φ(z) and σ(z) will be regarded as vectors of purely formal power series, and it will not be assumed that φ(z) is analytic or that the series for φ(z) or σ(z) converge. Nevertheless, because of a process developed by Cowen and MacCluer, if the given φ(z) represents a map of the unit ball into itself of an appropriate form, then the results of this paper can be used to produce solutions of Schröder's equation that are convergent power series, or (sometimes) to show that no such solution exists. A method of matrix completion is used. © 2007 Birkhäuser Verlag, Basel. N

## DOI

10.1007/s00010-007-2875-9

## First Page

26

## Last Page

61

## Publication Title

Aequationes Mathematicae

## Recommended Citation

Enoch, R. D. (2007). Formal power series solutions of Schröder’s equation. Aequationes Mathematicae 74(1-2): 26–61 (2007). https://doi.org/10.1007/s00010-007-2875-9