Isogeny classification conjecture for Denjoy systems

Let (DFk,Fk)(\mathcal{D}_{F_k},F_k), for k{0,1}k\in\{0,1\}, be Denjoy systems with invariants (ρk,Qk)(\rho_k,Q_k). For ATA\subset\mathbb{T}, write QAT\mathbb{Q}A\subset\mathbb{T} for the Q\mathbb{Q}-submodule generated by AA, and let \equiv denote equality of subsets of T\mathbb{T} up to rotations. Denjoy isogeny classification conjecture. The two Denjoy systems are isogenous if and only if there exists M=(mij)PGL2(Q)M=(m_{ij})\in\operatorname{PGL}_2(\mathbb{Q}) such that

ρ1=m11ρ0+m12m21ρ0+m22\rho_1=\frac{m_{11}\rho_0+m_{12}}{m_{21}\rho_0+m_{22}}

and

QQ11m21ρ0+m22QQ0.\mathbb{Q}Q_1\equiv\frac{1}{m_{21}\rho_0+m_{22}}\mathbb{Q}Q_0.

This is proposed as a classification of Denjoy systems up to isogeny, generalizing the proved Sturmian classification; no resolution is given for the general Denjoy case.

Sources & referencesView supporting material

Primary source

Scott Schmieding and Christopher-Lloyd Simon, “Isogenies of minimal Cantor systems: from Sturmian to Denjoy and interval exchanges”, arXiv:2503.02168 (2025).

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