Fibers of word maps and the multiplicities of nonabelian composition factors

Abstract

Call a reduced word $w$ multiplicity-bounding if and only if a finite group on which the word map of $w$ has a fiber of positive proportion $\rho$ can only contain each nonabelian finite simple group $S$ as a composition factor with multiplicity bounded in terms of $\rho$ and $S$. In this paper, based on recent work of Nikolov, we present methods to show that a given reduced word is multiplicity-bounding and apply them to give some nontrivial examples of multiplicity-bounding words, such as words of the form $x^e$, where $x$ is a single variable and $e$ an odd integer.

28 pages, 1 table; v2: some revisions necessitated by the author's discovery that the power word x^8 is NOT multiplicity-bounding (which was originally overlooked because of a programming error)