Element order versus minimal degree in permutation groups: an old lemma with new applications

László Babai, Ákos Seress

Published 2014-01-02Version 1

In this note we present a simplified and slightly generalized version of a lemma the authors published in 1987. The lemma as stated here asserts that if the order of a permutation of $n$ elements is greater than $n^{\alpha}$ then some non-identity power of the permutation has support size less than $n/{\alpha}$. The original version made an unnecessary additional assumption on the cycle structure of the permutation; the proof of the present cleaner version follows the original proof verbatim. Application areas include parallel and sequential algorithms for permutation groups, the diameter of Cayley graphs of permutation groups, and the automorphisms of structures with regularity constraints such as Latin squares, Steiner 2-designs, and strongly regular graphs. This note also serves as a modest tribute to the junior author whose untimely passing is deeply mourned.