Hensley's dimension-denominator conjecture for continued-fraction alphabets

Let AN\mathcal A\subset\mathbb N be a finite alphabet. Let CA\mathcal C_{\mathcal A} be the continued-fraction Cantor set

CA={[a1,a2,]:ajA for all j1},\mathcal C_{\mathcal A}=\{[a_1,a_2,\dots]:a_j\in\mathcal A\text{ for all }j\ge1\},

and let DA\mathcal D_{\mathcal A} be the set of denominators of the partial convergents to CA\mathcal C_{\mathcal A}. Denote the Hausdorff dimension of CA\mathcal C_{\mathcal A} by δA\delta_{\mathcal A}.

Hensley's conjecture. The denominators contain every sufficiently large natural number if and only if the Hausdorff dimension exceeds one half:

DAN1δA>1/2.\mathcal D_{\mathcal A}\supset\mathbb N_{\gg1}\qquad\Longleftrightarrow\qquad\delta_{\mathcal A}>1/2.

The source later states that this claim is false: the alphabet {2,4,6,8,10}\{2,4,6,8,10\} has dimension 0.517>1/20.517\dots>1/2 but has congruence obstructions. Thus the conjecture is refuted.

Sources & referencesView supporting material

Primary source

Alex Kontorovich, “From Apollonius To Zaremba: Local-Global Phenomena in Thin Orbits”, arXiv:1208.5460 (2012).

Additional references

2 papers in this index state this conjecture (2011–2012). The statement above is taken from the most recent of them; the others are arXiv:1107.3776.

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