Openness conjecture for proper and cocompact actions

From papers

Let G/HG/H be a homogeneous space of reductive type, and let Γ\Gamma be a discrete subgroup of GG acting properly discontinuously and cocompactly on G/HG/H. Write Hom(Γ,G)\operatorname{Hom}(\Gamma,G) for the space of representations of Γ\Gamma in GG. Openness conjecture. There is a neighborhood UHom(Γ,G)\mathcal{U}\subset\operatorname{Hom}(\Gamma,G) of the natural inclusion such that every ρU\rho\in\mathcal{U} is discrete and faithful, and the action of Γ\Gamma on G/HG/H via ρ\rho is properly discontinuous and cocompact. Stability under small deformations would give openness of the space of proper and cocompact actions, a phenomenon known in important special cases but posed here in this generality.

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Primary source

Fanny Kassel and Nicolas Tholozan, “Sharpness of proper and cocompact actions on reductive homogeneous spaces”, arXiv:2410.08179 (2026).

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