<p>A permutation <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="493_2024_127_Article_IEq1.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="51" /> </InlineMediaObject> <EquationSource Format="TEX">\(\pi \in \mathbb {S}_n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>π</mi> <mo>∈</mo> <msub> <mi mathvariant="double-struck">S</mi> <mi>n</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> is <i>k</i>-<i>balanced</i> if every permutation of order <i>k</i> occurs in <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="493_2024_127_Article_IEq2.gif" Format="GIF" Height="10" Rendition="HTML" Resolution="72" Type="Linedraw" Width="13" /> </InlineMediaObject> <EquationSource Format="TEX">\(\pi \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>π</mi> </math></EquationSource> </InlineEquation> equally often, through order-isomorphism. In this paper, we explicitly construct <i>k</i>-balanced permutations for <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="493_2024_127_Article_IEq3.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="42" /> </InlineMediaObject> <EquationSource Format="TEX">\(k \le 3\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>k</mi> <mo>≤</mo> <mn>3</mn> </mrow> </math></EquationSource> </InlineEquation>, and every <i>n</i> that satisfies the necessary divisibility conditions. In contrast, we prove that for <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="493_2024_127_Article_IEq4.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="42" /> </InlineMediaObject> <EquationSource Format="TEX">\(k \ge 4\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>k</mi> <mo>≥</mo> <mn>4</mn> </mrow> </math></EquationSource> </InlineEquation>, no such permutations exist. In fact, we show that in the case <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="493_2024_127_Article_IEq5.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="42" /> </InlineMediaObject> <EquationSource Format="TEX">\(k \ge 4\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>k</mi> <mo>≥</mo> <mn>4</mn> </mrow> </math></EquationSource> </InlineEquation>, every <i>n</i>-element permutation is at least <InlineEquation ID="IEq6"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="493_2024_127_Article_IEq6.gif" Format="GIF" Height="21" Rendition="HTML" Resolution="72" Type="Linedraw" Width="65" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Omega _n(n^{k-1})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="normal">Ω</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <msup> <mi>n</mi> <mrow> <mi>k</mi> <mo>-</mo> <mn>1</mn> </mrow> </msup> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> far from being <i>k</i>-balanced. This lower bound is matched for <InlineEquation ID="IEq7"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="493_2024_127_Article_IEq7.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="41" /> </InlineMediaObject> <EquationSource Format="TEX">\(k=4\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>k</mi> <mo>=</mo> <mn>4</mn> </mrow> </math></EquationSource> </InlineEquation>, by a construction based on the Erdős–Szekeres permutation.</p>

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How Balanced Can Permutations Be?

  • Gal Beniamini,
  • Nir Lavee,
  • Nati Linial

摘要

A permutation \(\pi \in \mathbb {S}_n\) π S n is k-balanced if every permutation of order k occurs in \(\pi \) π equally often, through order-isomorphism. In this paper, we explicitly construct k-balanced permutations for \(k \le 3\) k 3 , and every n that satisfies the necessary divisibility conditions. In contrast, we prove that for \(k \ge 4\) k 4 , no such permutations exist. In fact, we show that in the case \(k \ge 4\) k 4 , every n-element permutation is at least \(\Omega _n(n^{k-1})\) Ω n ( n k - 1 ) far from being k-balanced. This lower bound is matched for \(k=4\) k = 4 , by a construction based on the Erdős–Szekeres permutation.