Iterating sine, equivalence classes of variable changes, and groups with few conjugacy classes

TL;DR

This paper explores the dynamics of iterated smooth functions near zero and classifies conjugacy classes of formal variable changes, revealing connections to finite p-groups with minimal conjugacy classes.

Contribution

It provides a classification of conjugacy classes in formal change of variable groups and links these to finite p-groups with few conjugacy classes, expanding understanding of noncommutative group structures.

Findings

Asymptotic behavior of iterated functions near zero

Classification of conjugacy classes in formal series groups

Identification of p-groups with minimal conjugacy classes

Abstract

This is an expository paper about iterations of a smooth real function $f$ on $[0,\varepsilon)$ such that $f(0)=0$, $f'(0)=1$, and $f(x)<x$ for $x>0$, i.e., the sequence defined by $x_{n+1}=f(x_n)$. This sequence has interesting asymptotics, whose study leads to the question of classifying conjugacy classes in the group of formal changes of variable $y=f(x)$, i.e., formal series $f(x)=x+a_2x^2+a_3x^2+...$ with real coefficients (under composition). The same classification applies over a finite field $\mathbb{F}_p$ for suitably truncated series $f$, defining a family of $p$-groups which have the smallest number of conjugacy classes for a given order, i.e., are the ``most noncommutative" finite groups currently known. The paper should be accessible to undergraduates and at least partially to advanced high school students.