Chabauty Limits of Subgroups of $SL(n,{\mathbb{Q}}_p)$

Abstract

We study the Chabauty compactification of two families of closed subgroups of $SL(n,{\mathbb{Q}}_p)$. The first family is the set of all parahoric subgroups of $SL(n,{\mathbb{Q}}_p)$. Although the Chabauty compactification of parahoric subgroups is well studied, we give a different and more geometric proof using various Levi decompositions of $SL(n,{\mathbb{Q}}_p)$. Let $C$ be the subgroup of diagonal matrices in $SL(n,{\mathbb{Q}}_p)$. The second family is the set of all $SL(n,{\mathbb{Q}}_p)$-conjugates of $C$. We give a classification of the Chabauty limits of conjugates of $C$ using the action of $SL(n,{\mathbb{Q}}_p)$ on its associated Bruhat--Tits building and compute all of the limits for $n\le 4$ (up to conjugacy). In contrast, for $n\ge 7$ we prove there are infinitely many $SL(n,{\mathbb{Q}}_p)$-nonconjugate Chabauty limits of conjugates of $C$. Along the way we construct an explicit homeomorphism between the Chabauty compactification in $\mathfrak{sl}(n,{\mathbb{Q}}_p)$ of $SL(n,{\mathbb{Q}}_p)$-conjugates of the $p$-adic Lie algebra of $C$ and the Chabauty compactification of $SL(n,{\mathbb{Q}}_p)$-conjugates of $C$.

arXiv comment32 pages Arguments strengthened to remove assumption p does not divide n. (Theorems now true for all n.) Ideas added to sections 4,6 and 7. Alain Valette added as author

References in corpus (1)

• Limits Under Conjugacy of the Diagonal Subgroup in SL(n,R)

Cited by in corpus (1)

• On the Chabauty space of ${\text{PSL}}_2(\mathbb{R})$, I: lattices and grafting