The golden-ratio conjecture for radii of convergence of q-deformed real numbers

Let [x]q[x]_q denote the qq-deformation of a real number x>0x>0, viewed as a power series in the complex variable qq, and let R(x)R(x) be its radius of convergence. For the golden ratio φ=(1+5)/2\varphi=(1+\sqrt{5})/2, write R(φ)R(\varphi) for the radius of convergence of [φ]q[\varphi]_q; explicitly, R(φ)=(35)/2R(\varphi)=(3-\sqrt{5})/2. Golden-ratio radius conjecture. For every real x>0x>0, the radius of convergence satisfies

R(x)R(φ),R(x)\geq R(\varphi),

and equality holds only for those xx that are PSL(2,Z)\operatorname{PSL}(2,\mathbb{Z})-equivalent to φ\varphi. This conjecture identifies the golden ratio as the extremal case for the convergence radius of the qq-deformation; the supplied evidence states that the claim was proved for rational xx, while the corresponding assertion for irrational xx is not resolved here.

Sources & referencesView supporting material

Primary source

Sophie Morier-Genoud and Valentin Ovsienko, “q-deformed rationals and irrationals”, arXiv:2503.23834 (2025).

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