Orbit-closure dichotomy for Apollonian circle packings

Let P\mathcal P be an Apollonian circle packing with symmetry group Γ<PSL2(C)\Gamma<\operatorname{PSL}_2(\mathbb C). For any circle CCΛC\in \mathcal C_\Lambda^*, consider its orbit ΓC\Gamma C. Orbit-closure dichotomy. The orbit ΓC\Gamma C is either closed or dense in CΛ\mathcal C_\Lambda. This conjecture extends the orbit-closure classification known for geometrically finite acylindrical groups to the Apollonian group, which is not acylindrical because its compact core is a genus-two handlebody with compressible boundary. Whether the asserted dichotomy holds remains open.

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Primary source

Hee Oh, “Dynamics and Rigidity through the Lens of Circles”, arXiv:2510.10771 (2026).

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