Khintchine's theorem with a moving target

Let γ=(γq)q\gamma=(\gamma_q)_q be a sequence of real numbers and let ψ:N\tomathbbR0\psi:\mathbb{N}\tomathbb{R}_{\geq 0} be a function. Define

W(ψ,γ)={α[0,1]:qαγq<ψ(q) for infinitely many q\inmathbbN}.W(\psi,\gamma)=\{\alpha\in[0,1]:\|q\alpha-\gamma_q\|<\psi(q)\ \text{for infinitely many }q\inmathbb{N}\}.

Khintchine's moving-target conjecture. If ψ:N\tomathbbR0\psi:\mathbb{N}\tomathbb{R}_{\geq 0} is decreasing and q=1ψ(q)=\sum_{q=1}^{\infty}\psi(q)=\infty, then

m(W(ψ,γ))=1.m(W(\psi,\gamma))=1.

This asks whether the divergence part of the inhomogeneous Khintchine theorem remains valid when the target center depends on the denominator. The fixed-target theorem is classical, while the moving-target version is posed as an open problem and is not known in one dimension.

Sources & referencesView supporting material

Primary source

Gilbert Michaud and Felipe A. Ramírez, “Toward Khintchine's theorem with a moving target: extra divergence or finitely centered target”, arXiv:2506.04187 (2025).

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