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

Abstract

This is an expository paper about iterations of a smooth real function $f$ on $[0,\epsilon )$ such that $f(0)=0$, $f^{\prime }(0)=1$, 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 $f(x)=x+a_2{x}^2+a_3{x}^2+...$ 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.

9 pages, latex