<p>A family <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathcal {F}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">F</mi> </math></EquationSource> </InlineEquation> of subsets of <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq2.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="94" /> </InlineMediaObject> <EquationSource Format="TEX">\(\{1,2,\ldots ,n\}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">{</mo> <mn>1</mn> <mo>,</mo> <mn>2</mn> <mo>,</mo> <mo>…</mo> <mo>,</mo> <mi>n</mi> <mo stretchy="false">}</mo> </mrow> </math></EquationSource> </InlineEquation> is called a <i>t</i>-intersecting family if <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq3.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="90" /> </InlineMediaObject> <EquationSource Format="TEX">\(|F\cap G| \ge t\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">|</mo> <mi>F</mi> <mo>∩</mo> <mi>G</mi> <mo stretchy="false">|</mo> <mo>≥</mo> <mi>t</mi> </mrow> </math></EquationSource> </InlineEquation> for any two members <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq4.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="70" /> </InlineMediaObject> <EquationSource Format="TEX">\(F, G \in \mathcal {F}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>F</mi> <mo>,</mo> <mi>G</mi> <mo>∈</mo> <mi mathvariant="script">F</mi> </mrow> </math></EquationSource> </InlineEquation> and for some positive integer <i>t</i>. If <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq5.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="38" /> </InlineMediaObject> <EquationSource Format="TEX">\(t=1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>t</mi> <mo>=</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation>, then we call the family <InlineEquation ID="IEq6"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathcal {F}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">F</mi> </math></EquationSource> </InlineEquation> to be intersecting. Define the set <InlineEquation ID="IEq7"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq7.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="293" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathcal {I}(\mathcal {F}) = \{F\cap G: F, G \in \mathcal {F} \text { and } F \ne G\}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">I</mi> <mo stretchy="false">(</mo> <mi mathvariant="script">F</mi> <mo stretchy="false">)</mo> <mo>=</mo> <mo stretchy="false">{</mo> <mi>F</mi> <mo>∩</mo> <mi>G</mi> <mo>:</mo> <mi>F</mi> <mo>,</mo> <mi>G</mi> <mo>∈</mo> <mi mathvariant="script">F</mi> <mspace width="0.333333em" /> <mtext>and</mtext> <mspace width="0.333333em" /> <mi>F</mi> <mo>≠</mo> <mi>G</mi> <mo stretchy="false">}</mo> </mrow> </math></EquationSource> </InlineEquation> to be the collection of all distinct intersections of <InlineEquation ID="IEq8"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathcal {F}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">F</mi> </math></EquationSource> </InlineEquation>. Frankl et al. proved an upper bound for the size of <InlineEquation ID="IEq9"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq9.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="39" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathcal {I}(\mathcal {F})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">I</mi> <mo stretchy="false">(</mo> <mi mathvariant="script">F</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> of intersecting families <InlineEquation ID="IEq10"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathcal {F}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">F</mi> </math></EquationSource> </InlineEquation> of <i>k</i>-subsets of <InlineEquation ID="IEq11"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq2.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="94" /> </InlineMediaObject> <EquationSource Format="TEX">\(\{1,2,\ldots ,n\}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">{</mo> <mn>1</mn> <mo>,</mo> <mn>2</mn> <mo>,</mo> <mo>…</mo> <mo>,</mo> <mi>n</mi> <mo stretchy="false">}</mo> </mrow> </math></EquationSource> </InlineEquation>. Their theorem holds for integers <InlineEquation ID="IEq12"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq12.gif" Format="GIF" Height="18" Rendition="HTML" Resolution="72" Type="Linedraw" Width="66" /> </InlineMediaObject> <EquationSource Format="TEX">\(n \ge 50 k^2\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>50</mn> <msup> <mi>k</mi> <mn>2</mn> </msup> </mrow> </math></EquationSource> </InlineEquation>. In this article, we prove an upper bound for the size of <InlineEquation ID="IEq13"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq9.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="39" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathcal {I}(\mathcal {F})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">I</mi> <mo stretchy="false">(</mo> <mi mathvariant="script">F</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> of <i>t</i>-intersecting families <InlineEquation ID="IEq14"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">\(\mathcal {F}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">F</mi> </math></EquationSource> </InlineEquation>, provided that <i>n</i> exceeds a certain number <i>f</i>(<i>k</i>,&#xa0;<i>t</i>). Along the way we also improve the threshold <InlineEquation ID="IEq15"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq15.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">\(k^2\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>k</mi> <mn>2</mn> </msup> </math></EquationSource> </InlineEquation> to <InlineEquation ID="IEq16"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11139_2025_1081_Article_IEq16.gif" Format="GIF" Height="18" Rendition="HTML" Resolution="72" Type="Linedraw" Width="58" /> </InlineMediaObject> <EquationSource Format="TEX">\(k^{3/2+o(1)}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>k</mi> <mrow> <mn>3</mn> <mo stretchy="false">/</mo> <mn>2</mn> <mo>+</mo> <mi>o</mi> <mo stretchy="false">(</mo> <mn>1</mn> <mo stretchy="false">)</mo> </mrow> </msup> </math></EquationSource> </InlineEquation> for the intersecting families.</p>

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An improved threshold for the number of distinct intersections of intersecting families

  • Jagannath Bhanja,
  • Sayan Goswami

摘要

A family \(\mathcal {F}\) F of subsets of \(\{1,2,\ldots ,n\}\) { 1 , 2 , , n } is called a t-intersecting family if \(|F\cap G| \ge t\) | F G | t for any two members \(F, G \in \mathcal {F}\) F , G F and for some positive integer t. If \(t=1\) t = 1 , then we call the family \(\mathcal {F}\) F to be intersecting. Define the set \(\mathcal {I}(\mathcal {F}) = \{F\cap G: F, G \in \mathcal {F} \text { and } F \ne G\}\) I ( F ) = { F G : F , G F and F G } to be the collection of all distinct intersections of \(\mathcal {F}\) F . Frankl et al. proved an upper bound for the size of \(\mathcal {I}(\mathcal {F})\) I ( F ) of intersecting families \(\mathcal {F}\) F of k-subsets of \(\{1,2,\ldots ,n\}\) { 1 , 2 , , n } . Their theorem holds for integers \(n \ge 50 k^2\) n 50 k 2 . In this article, we prove an upper bound for the size of \(\mathcal {I}(\mathcal {F})\) I ( F ) of t-intersecting families \(\mathcal {F}\) F , provided that n exceeds a certain number f(kt). Along the way we also improve the threshold \(k^2\) k 2 to \(k^{3/2+o(1)}\) k 3 / 2 + o ( 1 ) for the intersecting families.