Finite groups with large Noether number are almost cyclic

Abstract

Noether, Fleischmann and Fogarty proved that if the characteristic of the underlying field does not divide the order $|G|$ of a finite group $G$, then the polynomial invariants of $G$ are generated by polynomials of degrees at most $|G|$. Let $\beta (G)$ denote the largest indispensable degree in such generating sets. Cziszter and Domokos recently described finite groups $G$ with $|G|/\beta (G)$ at most $2$. We prove an asymptotic extension of their result. Namely, $|G|/\beta (G)$ is bounded for a finite group $G$ if and only if $G$ has a characteristic cyclic subgroup of bounded index. In the course of the proof we obtain the following surprising result. If $S$ is a finite simple group of Lie type or a sporadic group then we have $\beta (S)\le {|S|}^{39/40}$. We ask a number of questions motivated by our results.

updated grant numbers