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Published online by Cambridge University Press: 16 August 2019
Let $\mathbf{H}_{\mathbb{H}}^{n}$ denote the
$n$-dimensional quaternionic hyperbolic space. The linear group
$\text{Sp}(n,1)$ acts on
$\mathbf{H}_{\mathbb{H}}^{n}$ by isometries. A subgroup
$G$ of
$\text{Sp}(n,1)$ is called Zariski dense if it neither fixes a point on
$\mathbf{H}_{\mathbb{H}}^{n}\cup \unicode[STIX]{x2202}\mathbf{H}_{\mathbb{H}}^{n}$ nor preserves a totally geodesic subspace of
$\mathbf{H}_{\mathbb{H}}^{n}$. We prove that a Zariski dense subgroup
$G$ of
$\text{Sp}(n,1)$ is discrete if for every loxodromic element
$g\in G$ the two-generator subgroup
$\langle f,gfg^{-1}\rangle$ is discrete, where the generator
$f\in \text{Sp}(n,1)$ is a certain fixed element not necessarily from
$G$.
The first author acknowledges partial support from SERB MATRICS grant MTR/2017/000355; the third author is supported by NBHM-SRF.