Definable rank, o-minimal groups, and Wiegold's problem

Abstract

We show that an o-minimal structure M defines groups with infinite definable rank if and only if M defines some finite power of $\mathbb{Q}/\mathbb{Z}$. If no interval of M is countable, then all groups definable in M have finite definable rank. In general, we prove that every definable group $G$ in an arbitrary o-minimal structure is an extension of a definable periodic group $P$ by a (maximal unique) definably connected definably finitely generated subgroup $\hat{G}$. When $G$ is definably connected, $P$ is abelian and the extension almost split, in that $G$ is an almost direct product $G=(\hat{G}\times P)/F$, for some finite central subgroup $F$. The definable rank of $\hat{G}$ is bounded above by its dimension, and the upper bound is strict whenever $\hat{G}$ is not solvable. Along the way, we show that every linear definable group has finite definable rank. This provides another proof, and a generalization to linear o-minimal groups, of the fact that linear algebraic groups over an algebraically closed field of characteristic $0$ contain a Zariski-dense finitely generated subgroup. We further prove that every perfect definable group is normally monogenic, generalizing the finite group case. This yields a positive answer to Wiegold's problem in the o-minimal setting.

v2: paper completely rewritten with corrections and several new results. To be published in Isr. J. Math