Published online by Cambridge University Press: 06 July 2018
Let $R$ be a ring of characteristic
$0$ with field of fractions
$K$ and let
$m\geq 2$. The Böttcher coordinate of a power series
$\unicode[STIX]{x1D711}(x)\in x^{m}+x^{m+1}R\unicode[STIX]{x27E6}x\unicode[STIX]{x27E7}$ is the unique power series
$f_{\unicode[STIX]{x1D711}}(x)\in x+x^{2}K\unicode[STIX]{x27E6}x\unicode[STIX]{x27E7}$ satisfying
$\unicode[STIX]{x1D711}\circ f_{\unicode[STIX]{x1D711}}(x)=f_{\unicode[STIX]{x1D711}}(x^{m})$. In this paper we study the integrality properties of the coefficients of
$f_{\unicode[STIX]{x1D711}}(x)$, partly for their intrinsic interest and partly for potential applications to
$p$-adic dynamics. Results include: (1) if
$p$ is prime and
$R=\mathbb{Z}_{p}$ and
$\unicode[STIX]{x1D711}(x)\in x^{p}+px^{p+1}R\unicode[STIX]{x27E6}x\unicode[STIX]{x27E7}$, then
$f_{\unicode[STIX]{x1D711}}(x)\in R\unicode[STIX]{x27E6}x\unicode[STIX]{x27E7}$. (2) If
$\unicode[STIX]{x1D711}(x)\in x^{m}+mx^{m+1}R\unicode[STIX]{x27E6}x\unicode[STIX]{x27E7}$, then
$f_{\unicode[STIX]{x1D711}}(x)=x\sum _{k=0}^{\infty }a_{k}x^{k}/k!$ with all
$a_{k}\in R$. (3) In (2), if
$m=p^{2}$, then
$a_{k}\equiv -1~\text{(mod}~p\text{)}$ for all
$k$ that are powers of
$p$.