Thompson's group $F$ is almost $\frac{3}{2}$-generated

Abstract

Recall that a group $G$ is said to be $\frac{3}{2}$-generated if every non-trivial element of $G$ belongs to a generating pair of $G$. Thompson's group $V$ was proved to be $\frac{3}{2}$-generated by Donoven and Harper in 2019. It was the first example of an infinite finitely presented non-cyclic $\frac{3}{2}$-generated group. Recently, Bleak, Harper and Skipper proved that Thompson's group $T$ is also $\frac{3}{2}$-generated. In this paper, we prove that Thompson's group $F$ is "almost" $\frac{3}{2}$-generated in the sense that every element of $F$ whose image in the abelianization forms part of a generating pair of ${\mathbb{Z}}^2$ is part of a generating pair of $F$. We also prove that for every non-trivial element $f\in F$ there is an element $g\in F$ such that the subgroup $⟨f,g⟩$ contains the derived subgroup of $F$. Moreover, if $f$ does not belong to the derived subgroup of $F$, then there is an element $g\in F$ such that $⟨f,g⟩$ has finite index in $F$.

15 pages, 1 figure