<p>A <InlineEquation ID="IEq11"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq11.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="13" /> </InlineMediaObject> <EquationSource Format="TEX">\(k\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>k</mi> </math></EquationSource> </InlineEquation>-uniform family <InlineEquation ID="IEq12"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq12.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> is called intersecting if <InlineEquation ID="IEq13"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq13.gif" Format="GIF" Height="18" Rendition="HTML" Resolution="72" Type="Linedraw" Width="85" /> </InlineMediaObject> <EquationSource Format="TEX">\(F\cap F'\neq \emptyset\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>F</mi> <mo>∩</mo> <msup> <mi>F</mi> <mo>′</mo> </msup> <mo>≠</mo> <mi mathvariant="normal">∅</mi> </mrow> </math></EquationSource> </InlineEquation> for all <InlineEquation ID="IEq14"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq14.gif" Format="GIF" Height="18" Rendition="HTML" Resolution="72" Type="Linedraw" Width="74" /> </InlineMediaObject> <EquationSource Format="TEX">\(F,F'\in \mathcal{F}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>F</mi> <mo>,</mo> <msup> <mi>F</mi> <mo>′</mo> </msup> <mo>∈</mo> <mi mathvariant="script">F</mi> </mrow> </math></EquationSource> </InlineEquation>. The shadow family <InlineEquation ID="IEq15"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq15.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="27" /> </InlineMediaObject> <EquationSource Format="TEX">\(\partial \mathcal{F}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>∂</mi> <mi mathvariant="script">F</mi> </mrow> </math></EquationSource> </InlineEquation> is the family of <InlineEquation ID="IEq16"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq16.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="52" /> </InlineMediaObject> <EquationSource Format="TEX">\((k-1)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <mi>k</mi> <mo>-</mo> <mn>1</mn> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>-element sets that are contained in some members of <InlineEquation ID="IEq17"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq12.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>. The shadow degree (or minimum positive co-degree) of <InlineEquation ID="IEq18"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq12.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> is defined as the maximum integer <InlineEquation ID="IEq19"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq19.gif" Format="GIF" Height="10" Rendition="HTML" Resolution="72" Type="Linedraw" Width="11" /> </InlineMediaObject> <EquationSource Format="TEX">\(r\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>r</mi> </math></EquationSource> </InlineEquation> such that every <InlineEquation ID="IEq20"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq20.gif" Format="GIF" Height="15" Rendition="HTML" Resolution="72" Type="Linedraw" Width="61" /> </InlineMediaObject> <EquationSource Format="TEX">\(E\in \partial \mathcal{F}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>E</mi> <mo>∈</mo> <mi>∂</mi> <mi mathvariant="script">F</mi> </mrow> </math></EquationSource> </InlineEquation> is contained in at least <InlineEquation ID="IEq21"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq19.gif" Format="GIF" Height="10" Rendition="HTML" Resolution="72" Type="Linedraw" Width="11" /> </InlineMediaObject> <EquationSource Format="TEX">\(r\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>r</mi> </math></EquationSource> </InlineEquation> members of <InlineEquation ID="IEq22"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq12.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>. Balogh, Lemons and Palmer [1] determined the maximum size of an intersecting <InlineEquation ID="IEq23"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq11.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="13" /> </InlineMediaObject> <EquationSource Format="TEX">\(k\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>k</mi> </math></EquationSource> </InlineEquation>-uniform family with shadow degree at least <InlineEquation ID="IEq24"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq19.gif" Format="GIF" Height="10" Rendition="HTML" Resolution="72" Type="Linedraw" Width="11" /> </InlineMediaObject> <EquationSource Format="TEX">\(r\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>r</mi> </math></EquationSource> </InlineEquation> for <InlineEquation ID="IEq25"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq25.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="88" /> </InlineMediaObject> <EquationSource Format="TEX">\(n\geq n_0(k,r)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <msub> <mi>n</mi> <mn>0</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>k</mi> <mo>,</mo> <mi>r</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, where <InlineEquation ID="IEq26"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq26.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="55" /> </InlineMediaObject> <EquationSource Format="TEX">\(n_0(k,r)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>n</mi> <mn>0</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>k</mi> <mo>,</mo> <mi>r</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is doubly exponential in <InlineEquation ID="IEq27"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq11.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="13" /> </InlineMediaObject> <EquationSource Format="TEX">\(k\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>k</mi> </math></EquationSource> </InlineEquation> for <InlineEquation ID="IEq28"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq28.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="75" /> </InlineMediaObject> <EquationSource Format="TEX">\(4\leq r\leq k\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>4</mn> <mo>≤</mo> <mi>r</mi> <mo>≤</mo> <mi>k</mi> </mrow> </math></EquationSource> </InlineEquation>. In the present paper, we present a short proof of this result for <InlineEquation ID="IEq29"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq29.gif" Format="GIF" Height="33" Rendition="HTML" Resolution="72" Type="Linedraw" Width="127" /> </InlineMediaObject> <EquationSource Format="TEX">\(n\geq 2\frac{(r+1)^r}{\binom{2r-1}{r}}k\binom{2k}{k-1}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>2</mn> <mfrac> <msup> <mrow> <mo stretchy="false">(</mo> <mi>r</mi> <mo>+</mo> <mn>1</mn> <mo stretchy="false">)</mo> </mrow> <mi>r</mi> </msup> <mfenced close=")" open="("> <mfrac linethickness="0pt"> <mrow> <mn>2</mn> <mi>r</mi> <mo>-</mo> <mn>1</mn> </mrow> <mi>r</mi> </mfrac> </mfenced> </mfrac> <mi>k</mi> <mfenced close=")" open="("> <mfrac linethickness="0pt"> <mrow> <mn>2</mn> <mi>k</mi> </mrow> <mrow> <mi>k</mi> <mo>-</mo> <mn>1</mn> </mrow> </mfrac> </mfenced> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq30"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10474_2025_1526_Article_IEq28.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="75" /> </InlineMediaObject> <EquationSource Format="TEX">\(4\leq r\leq k\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>4</mn> <mo>≤</mo> <mi>r</mi> <mo>≤</mo> <mi>k</mi> </mrow> </math></EquationSource> </InlineEquation>.</p>

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Intersecting families with large shadow degree

  • P. Frankl,
  • J. Wang

摘要

A \(k\) k -uniform family \(\mathcal{F}\) F is called intersecting if \(F\cap F'\neq \emptyset\) F F for all \(F,F'\in \mathcal{F}\) F , F F . The shadow family \(\partial \mathcal{F}\) F is the family of \((k-1)\) ( k - 1 ) -element sets that are contained in some members of \(\mathcal{F}\) F . The shadow degree (or minimum positive co-degree) of \(\mathcal{F}\) F is defined as the maximum integer \(r\) r such that every \(E\in \partial \mathcal{F}\) E F is contained in at least \(r\) r members of \(\mathcal{F}\) F . Balogh, Lemons and Palmer [1] determined the maximum size of an intersecting \(k\) k -uniform family with shadow degree at least \(r\) r for \(n\geq n_0(k,r)\) n n 0 ( k , r ) , where \(n_0(k,r)\) n 0 ( k , r ) is doubly exponential in \(k\) k for \(4\leq r\leq k\) 4 r k . In the present paper, we present a short proof of this result for \(n\geq 2\frac{(r+1)^r}{\binom{2r-1}{r}}k\binom{2k}{k-1}\) n 2 ( r + 1 ) r 2 r - 1 r k 2 k k - 1 and \(4\leq r\leq k\) 4 r k .