Whirling injections, surjections, and other functions between finite sets

Michael Joseph, James Propp, Tom Roby

Published 2017-11-07Version 1

This paper analyzes a certain action called "whirling" that can be defined on any family of functions between two finite sets equipped with a linear (or cyclic) ordering. As a map on injections and surjections, we prove that within any whirling-orbit, any two elements of the codomain appear as outputs of functions the same number of times. This result, can be stated in terms of the homomesy phenomenon, which occurs when a statistic has the same average across every orbit. We further explore whirling on parking functions, order-preserving maps, and restricted growth words, discussing homomesy results for each case.