<p>The Jarník–Besicovitch theorem is a fundamental result in metric number theory which concerns the Hausdorff dimension for certain limsup sets. We discuss the analogous problem for liminf sets. Consider an infinite sequence of positive integers, <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3841_Article_IEq1.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="91" /> </InlineMediaObject> <EquationSource Format="TEX">\(S=\{q_{n}\}_{n\in {\mathbb {N}}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>S</mi> <mo>=</mo> <msub> <mrow> <mo stretchy="false">{</mo> <msub> <mi>q</mi> <mi>n</mi> </msub> <mo stretchy="false">}</mo> </mrow> <mrow> <mi>n</mi> <mo>∈</mo> <mi mathvariant="double-struck">N</mi> </mrow> </msub> </mrow> </math></EquationSource> </InlineEquation>, exhibiting exponential growth. For a given <i>n</i>-tuple of functions denoted as <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3841_Article_IEq2.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="131" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Psi :=~(\psi _1, \ldots ,\psi _n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="normal">Ψ</mi> <mo>:</mo> <mo>=</mo> <mspace width="3.33333pt" /> <mo stretchy="false">(</mo> <msub> <mi>ψ</mi> <mn>1</mn> </msub> <mo>,</mo> <mo>…</mo> <mo>,</mo> <msub> <mi>ψ</mi> <mi>n</mi> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, each of the form <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3841_Article_IEq3.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="88" /> </InlineMediaObject> <EquationSource Format="TEX">\(\psi _{i}(q)=q^{-\tau _{i}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>ψ</mi> <mi>i</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>q</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <msup> <mi>q</mi> <mrow> <mo>-</mo> <msub> <mi>τ</mi> <mi>i</mi> </msub> </mrow> </msup> </mrow> </math></EquationSource> </InlineEquation> for <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3841_Article_IEq4.gif" Format="GIF" Height="20" Rendition="HTML" Resolution="72" Type="Linedraw" Width="121" /> </InlineMediaObject> <EquationSource Format="TEX">\((\tau _{1},\dots ,\tau _{n})\in {\mathbb {R}}^{n}_{+}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mo stretchy="false">(</mo> <msub> <mi>τ</mi> <mn>1</mn> </msub> <mo>,</mo> <mo>⋯</mo> <mo>,</mo> <msub> <mi>τ</mi> <mi>n</mi> </msub> <mo stretchy="false">)</mo> </mrow> <mo>∈</mo> <msubsup> <mrow> <mi mathvariant="double-struck">R</mi> </mrow> <mo>+</mo> <mi>n</mi> </msubsup> </mrow> </math></EquationSource> </InlineEquation>, we calculate the Hausdorff dimension of the set of points that can be <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3841_Article_IEq5.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="14" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Psi \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Ψ</mi> </math></EquationSource> </InlineEquation>-approximated for all sufficiently large <InlineEquation ID="IEq6"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="209_2025_3841_Article_IEq6.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="45" /> </InlineMediaObject> <EquationSource Format="TEX">\(q\in S\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>q</mi> <mo>∈</mo> <mi>S</mi> </mrow> </math></EquationSource> </InlineEquation>. We prove this result in the generalised setting of approximation by abstract rationals as recently introduced by Fraser, Koivusalo, and Ramirez (Bull. Lond. Math. Soc. 2023). Some of the examples of this setting include the real weighted inhomogeneous approximation, <i>p</i>-adic weighted approximation, Diophantine approximation over complex numbers, and approximation on missing digit sets.</p>

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Liminf approximation sets for abstract rationals

  • Mumtaz Hussain,
  • Benjamin Ward

摘要

The Jarník–Besicovitch theorem is a fundamental result in metric number theory which concerns the Hausdorff dimension for certain limsup sets. We discuss the analogous problem for liminf sets. Consider an infinite sequence of positive integers, \(S=\{q_{n}\}_{n\in {\mathbb {N}}}\) S = { q n } n N , exhibiting exponential growth. For a given n-tuple of functions denoted as \(\Psi :=~(\psi _1, \ldots ,\psi _n)\) Ψ : = ( ψ 1 , , ψ n ) , each of the form \(\psi _{i}(q)=q^{-\tau _{i}}\) ψ i ( q ) = q - τ i for \((\tau _{1},\dots ,\tau _{n})\in {\mathbb {R}}^{n}_{+}\) ( τ 1 , , τ n ) R + n , we calculate the Hausdorff dimension of the set of points that can be \(\Psi \) Ψ -approximated for all sufficiently large \(q\in S\) q S . We prove this result in the generalised setting of approximation by abstract rationals as recently introduced by Fraser, Koivusalo, and Ramirez (Bull. Lond. Math. Soc. 2023). Some of the examples of this setting include the real weighted inhomogeneous approximation, p-adic weighted approximation, Diophantine approximation over complex numbers, and approximation on missing digit sets.