# Linearizing a power series by conjugation

Let $\mathfrak{I}:=\big\{ \, f:=\sum_{k=0}^\infty f_k z^k \in\mathbb{C}[[z]]\; : \text{s.t. }\; f_0=0 \;\text{ and }\; f_1=1\big\}$. A most basic result about linearization states that, for any $f\in\mathfrak{I}$ and for any $\lambda\in\mathbb{C}$ not a root of unity, there exists a unique $h\in\mathfrak{I}$ that linearizes $f(\lambda z)$: $$h(\lambda z)=f(\lambda h(z)).$$ Existence and uniqueness of this conjugation in the setting of formal power series is indeed easily established, as the coefficients of $h$ can be determined inductively from the data $(f_1,f_2,\dots)$ expanding the composition: one finds $$h_1=1$$ $$h_{n+1}={1\over \lambda^{n}-1 } \sum_{k\ge2,\,j} f_k \lambda^{k-1} h_{j_1}\dots h_{j_k},$$ the latter sum being extended over all integers $k\ge2$ and multi-indices $j\in\mathbb{Z}_+^k$ of length $|j|:=j_1+\dots+j_k=n+1$, thus with all components $j_i\le n$. The above recursion implies that the coefficients $h_n$ should be rational functions of $\lambda$, with denominators in the form $\prod_{k=1}^{n-1}(\lambda^k-1)^{m_{n,k}}$, where the exponents $m_{k,n}$ may be larger than $1$, at least at a first glance. However, after experiments with various $f$ and up to $n=50$ I observed that the form of $h_n$ always shows the simpler denominator, with all $m_{k,n}=1$: $$h_n={P_n(\lambda)\over (\lambda-1)(\lambda^2-1)\dots(\lambda^{n-1}-1)},$$ for some polynomials $P_n(\lambda)$ of degree at most ${n\choose 2}$.

Is this really a general phenomenon? If so, why? Is there a simpler formula (maybe still recursive) for $h_n$ than the above recursion?

You may find useful information in a recent article by D. Sauzin and al. "Explicit linearization of one-dimensional germs through tree-expansions" here, where they use "mould calculus" (introduced by J. Écalle 40 years ago) to write down the coefficients $h_n$ and explore the combinaotrial structure of their family.
I think it is very unusual for the $h_n$s to have a simpler formulation. Even in the case of a quadratic polynomial, the conjugating function $h$ need not be a polynomial. I'm afraid I don't have a general explanation of that for you, other than noticing it by working through some examples.
There is however a simpler construction of $h$ as a limit, rather than as a power series that works when $|\lambda|\neq 1$, namely let $$h_n=\lambda^{-n}\circ f^n.$$ Then, $h_n\to h$. This formulation is fairly well known so, perhaps, you are already aware of it. It's quite easy to work with programmatically.
Note, however that the case $|\lambda|=1$ is more subtle than you've stated. The existence of the linearization is then equivalent to the the fixed point being in the Fatou set. Indeed, if $\lambda$ is a root of unity, the fixed point is in the Julia set but, even if $\lambda$ is not a root of unity the fixed point might still be in the Julia set.
• Thank you, yes, I am aware of what you are saying. Here I only look at the formal series expansion, with no issue of convergence, so the only condition on $\lambda$ is not being a root of unity. (However one can assume $|\lambda|\neq 1$ if useful). May 12 '17 at 4:59
• How does this address the question? As I understand it was not conjectured that $h$ is a polynomial. The question was about denominators of the coefficients of $h$. May 12 '17 at 5:59