Hausdorff measure Duffin–Schaeffer conjecture

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Let k∈Nk\in\mathbb{N}, let ff be a dimension function such that r−kf(r)r^{-k}f(r) is monotonic, and let Ak”(ψ)⊂[0,1]k\mathcal{A}_k”(\psi)\subset[0,1]^k be the set of simultaneously reduced ψ\psi-well approximable points. Let φ\varphi denote Euler's totient function and let Hf\mathcal{H}^f be the Hausdorff ff-measure. Hausdorff measure Duffin–Schaeffer conjecture. For every approximating function ψ:N→R≥0\psi:\mathbb{N}\to\mathbb{R}_{\geq 0},

Hf(Ak”(ψ))={0if ∑q=1∞f(ψ(q)q)φ(q)k<∞,Hf([0,1]k)if ∑q=1∞f(ψ(q)q)φ(q)k=∞.\mathcal{H}^f(\mathcal{A}_k”(\psi))=\begin{cases}0&\text{if }\displaystyle\sum_{q=1}^{\infty}f\left(\frac{\psi(q)}q\right)\varphi(q)^k<\infty,\\\\[2ex]\mathcal{H}^f([0,1]^k)&\text{if }\displaystyle\sum_{q=1}^{\infty}f\left(\frac{\psi(q)}q\right)\varphi(q)^k=\infty. \end{cases}

This is the Hausdorff-measure generalisation of the one-dimensional and higher-dimensional Duffin–Schaeffer conjectures presented by Beresnevich and Velani; the supplied text does not state whether it has been resolved in this full form.

References

Primary source

Demi Allen and Edouard Daviaud, “A survey of recent extensions and generalisations of the Mass Transference Principle”, arXiv:2306.15535 (2023).

Additional references

3 papers in this index state this conjecture (2008–2023). The statement above is taken from the most recent of them; the others are arXiv:1704.06628, arXiv:0811.1234.

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