Balmer comparison-map conjecture for equivariant cell categories

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Let GG be a finite group, let \CellG\Cell{G} be the category of GG-cell algebras, and let \Spc(\CellGc)\Spc(\Cell{G}^c) be its tensor-triangular spectrum. The tensor unit is the trivial GG-C*-algebra, with endomorphism ring End⁡(\unit)=\Rep(G)\operatorname{End}(\unit)=\Rep(G), and there is a canonical comparison map

ρ ⁣:\Spc(\CellGc)⟶Spec⁡(\Rep(G)).\rho\colon \Spc(\Cell{G}^c)\longrightarrow \operatorname{Spec}(\Rep(G)).

Balmer's comparison-map conjecture. For every finite group GG, the comparison map is a homeomorphism

\Spc(\CellGc)≅Spec⁡(\Rep(G)).\Spc(\Cell{G}^c)\cong \operatorname{Spec}(\Rep(G)).

The map is known to be surjective for finite groups, and the paper proves that it is a homeomorphism when every nontrivial element of GG has prime order. Its being a homeomorphism for arbitrary finite groups remains open and is an obstacle to proving general countable stratification.

References

Primary source

Ivo Dell'Ambrogio and Rubén Martos, “Stratification in equivariant Kasparov theory”, arXiv:2412.21109 (2026).

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