Openness conjecture for proper and cocompact actions

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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 U⊂Hom⁡(Γ,G)\mathcal{U}\subset\operatorname{Hom}(\Gamma,G) of the natural inclusion such that every ρ∈U\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.

References

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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