Cyclic Sieving of Matchings

Abstract: The cyclic sieving phenomenon (CSP) was introduced by Reiner, Stanton, and White to study combinatorial structures with actions of cyclic groups. The crucial step is to find a polynomial, for example a q-analog, that satisfies the CSP conditions for an action. This polynomial will give us a lot of information about the symmetry and structure of the set under the action. In this paper, we study the cyclic sieving phenomenon of the cyclic group $C_{2n}$ acting on $P_{n,k}$, which is the set of matchings of $2n$ points on a circle with $k$ crossings. The noncrossing matchings ($k=0$) was recently studied as a Catalan object. In this paper, we study more general cases, the matchings with more number of crossings. We prove that there exists $q$-analog polynomials $f_{n,k}(q)$ such that $(P_{n,k},f_{n,k},C_{2n})$ exhibits the cyclic sieving phenomenon for $k=1,2,3$. In the proof, we also introduce an efficient representation of the elements in $P_{n,k}$, which helps us to understand the symmetrical structure of the set.