Approximate roots

Abstract

Given an integral domain $A$, a monic polynomial $P$ of degree $n$ with coefficients in $A$ and a divisor $p$ of $n$, invertible in $A$, there is a unique monic polynomial $Q$ such that the degree of $P-Q^{p}$ is minimal for varying $Q$. This $Q$, whose $p$-th power best approximates $P$, is called the $p$-th approximate root of $P$. If $f \in \mathbf{C}[[X]][Y]$ is irreducible, there is a sequence of characteristic approximate roots of $f$, whose orders are given by the singularity structure of $f$. This sequence gives important information about this singularity structure. We study its properties in this spirit and we show that most of them hold for the more general concept of semiroot. We show then how this local study adapts to give a proof of Abhyankar-Moh's embedding line theorem.