On Integral Forms of Specht Modules Labelled by Hook Partitions

Abstract: We investigate integral forms of simple modules of symmetric groups over fields of characteristic $0$ labelled by hook partitions. Building on work of Plesken and Craig, for every odd prime $p$, we give a set of representatives of the isomorphism classes of $\mathbb{Z}_p$-forms of the simple $\mathbb{Q}_p \mathfrak{S}_n$-module labelled by the partition $(n-k,1^k)$, where $n\in\mathbb{N}$ and $0\leq k\leq n-1$. We also settle the analogous question for $p=2$, assuming that $n\not\equiv 0\pmod{4}$ and $k\in{2,n-3}$. As a consequence this leads to a set of representatives of the isomorphism classes of $\mathbb{Z}$-forms of the simple $\mathbb{Q}\mathfrak{S}_n$-modules labelled by $(n-2,1^2)$ and $(3,1^{n-3})$, again assuming $n\not\equiv 0\pmod{4}$.