Average Size of a Self-conjugate (s, t)-Core Partition

Abstract

Armstrong, Hanusa and Jones conjectured that if $s,t$ are coprime integers, then the average size of an $(s,t)$-core partition and the average size of a self-conjugate $(s,t)$-core partition are both equal to $\frac {(s+t+1)(s-1)(t-1)}{24}$. Stanley and Zanello showed that the average size of an $(s,s+1)$-core partition equals $\binom {s+1}{3}/2$. Based on a bijection of Ford, Mai and Sze between self-conjugate $(s,t)$-core partitions and lattice paths in an $\lfloor \frac {s}{2} \rfloor \times \lfloor \frac {t}{2}\rfloor$ rectangle, we obtain the average size of a self-conjugate $(s,t)$-core partition as conjectured by Armstrong, Hanusa and Jones.