Closed expressions for averages of set partition statistics

Bobbie Chern, Persi Diaconis, Daniel M. Kane, Robert C. Rhoades

Published 2013-04-16, updated 2013-06-10Version 2

In studying the enumerative theory of super characters' of the group of upper triangular matrices over a finite field we found that the moments (mean, variance and higher moments) of novel statistics on set partitions have simple closed expressions as linear combinations of shifted bell numbers. It is shown here that families of other statistics have similar moments. The coefficients in the linear combinations are polynomials in $n$. This allows exact enumeration of the moments for small $n$ to determine exact formulae for all $n$.