Symplectic extension graphs and Ribet's lemma

Abstract

Let $p$ be an odd prime, and let $ρ\colon G\to \GSp_{2n}(\Z_p)$ be an irreducible symplectic residually multiplicity free representation. We define the symplectic extension graph $Γ_ρ^\GSp$ associated with $ρ$ to be the directed graph whose vertices are the residual factors of $ρ$ and a directed edge between two vertices corresponds to a nonsplit extension between the vertices realized by $ρ$. Using the Euclidean Bruhat--Tits building associated with $\GSp_{2n}(\Q_p)$, under some assumptions, we prove the existence of certain paths in $Γ_ρ^\GSp$ and show that $Γ_ρ^\GSp$ is a disjoint union of nonempty strongly connected subgraphs. Moreover, we provide examples to show that the result we obtain is optimal.