Cores of partitions in rectangles

Abstract

For a positive integer $t\ge 2$, the $t$-core of a partition plays an important role in modular representation theory and combinatorics. We initiate the study of $t$-cores of partitions contained in an $r\times s$ rectangle. Our main results are as follows. We first give a simple formula for the number of partitions in the rectangle that are themselves $t$-cores and compute its asymptotics for large $r,s$. We then prove that the number of partitions inside the rectangle whose $t$-cores are a fixed partition $\ddot{I}$ is given by a product of binomial coefficients. Finally, we use this formula to compute the distribution of the $t$-core of a uniformly random partition inside the rectangle extending our previous work on all partitions of a fixed integer $n$ (Ann. Appl. Prob. 2023). In particular, we show that in the limit as $r,s\to \infty$ maintaining a fixed aspect ratio, we again obtain a Gamma distribution with the same shape parameter $\hat{I}\pm =(t-1)/2$ and rate parameter ${\hat{I}}^2$ that depends on the aspect ratio.

16 pages, 1 figure, minor improvements, final version