Mullineux map: $d$-balanced partitions and $d$-runner matrices

Abstract

Let $d,e>1$ be two integers. For $e$ prime, the Mullineux map $m_e$ describes tensor products of the irreducible modules of symmetric groups with the sign in characteristic $e$ as well as certain entries of decomposition matrices. Motivated by understanding new columns of decomposition matrices, we prove that if $λ$ is an $e$-regular partition such that $d$ divides the arm length of any rim hook of $λ$ of size divisible by $e$, then $m_e(λ)'$ is a partition such that the arm length of any of its rim hooks of size divisible by $e$ is congruent to $-1$ modulo $d$. We introduce a new parameter for partitions called the $d$-runner matrix and show that if $λ$ is as above, then the $d$-runner matrices of $λ$ and $m_e(λ)'$ agree. This determines $m_e(λ)'$ uniquely. We approach the whole problem combinatorially and take advantage of a new Abacus Mullineux Algorithm introduced in this paper. We also establish equivalent descriptions of the above partitions which provide an alternative version of the main result about the Mullineux map that becomes particularly strong when $d=2$.