<p>Let <i>V</i> be an <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\((n+\ell )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <mi>n</mi> <mo>+</mo> <mi>ℓ</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>-dimensional vector space over a finite field, and <i>W</i> a fixed <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\ell \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>ℓ</mi> </math></EquationSource> </InlineEquation>-dimensional subspace of <i>V</i>. Write <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\({V\brack n,0}\)</EquationSource> <EquationSource Format="MATHML"><math> <mfenced close="]" open="["> <mfrac linethickness="0pt"> <mi>V</mi> <mrow> <mi>n</mi> <mo>,</mo> <mn>0</mn> </mrow> </mfrac> </mfenced> </math></EquationSource> </InlineEquation> to be the set of all <i>n</i>-dimensional subspaces <i>U</i> of <i>V</i> satisfying <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\dim (U\cap W)=0\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>dim</mo> <mo stretchy="false">(</mo> <mi>U</mi> <mo>∩</mo> <mi>W</mi> <mo stretchy="false">)</mo> <mo>=</mo> <mn>0</mn> </mrow> </math></EquationSource> </InlineEquation>. A family <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\mathcal {F}\subseteq {V\brack n,0}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">F</mi> <mo>⊆</mo> <mfenced close="]" open="["> <mfrac linethickness="0pt"> <mi>V</mi> <mrow> <mi>n</mi> <mo>,</mo> <mn>0</mn> </mrow> </mfrac> </mfenced> </mrow> </math></EquationSource> </InlineEquation> is <i>t</i>-intersecting if <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\dim (A\cap B)\ge t\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>dim</mo> <mo stretchy="false">(</mo> <mi>A</mi> <mo>∩</mo> <mi>B</mi> <mo stretchy="false">)</mo> <mo>≥</mo> <mi>t</mi> </mrow> </math></EquationSource> </InlineEquation> for all <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(A,B\in \mathcal {F}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>A</mi> <mo>,</mo> <mi>B</mi> <mo>∈</mo> <mi mathvariant="script">F</mi> </mrow> </math></EquationSource> </InlineEquation>. A <i>t</i>-intersecting family <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(\mathcal {F}\subseteq {V\brack n,0}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">F</mi> <mo>⊆</mo> <mfenced close="]" open="["> <mfrac linethickness="0pt"> <mi>V</mi> <mrow> <mi>n</mi> <mo>,</mo> <mn>0</mn> </mrow> </mfrac> </mfenced> </mrow> </math></EquationSource> </InlineEquation> is called non-trivial if <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\dim (\cap _{F\in \mathcal {F}}F)&lt;t\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>dim</mo> <mo stretchy="false">(</mo> <msub> <mo>∩</mo> <mrow> <mi>F</mi> <mo>∈</mo> <mi mathvariant="script">F</mi> </mrow> </msub> <mi>F</mi> <mo stretchy="false">)</mo> <mo>&lt;</mo> <mi>t</mi> </mrow> </math></EquationSource> </InlineEquation>. In this paper, we describe the structure of non-trivial <i>t</i>-intersecting families of <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\({V\brack n,0}\)</EquationSource> <EquationSource Format="MATHML"><math> <mfenced close="]" open="["> <mfrac linethickness="0pt"> <mi>V</mi> <mrow> <mi>n</mi> <mo>,</mo> <mn>0</mn> </mrow> </mfrac> </mfenced> </math></EquationSource> </InlineEquation> with large size. In particular, we show the structure of the non-trivial <i>t</i>-intersecting families with maximum size, which extends the Hilton–Milner theorem for <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\({V\brack n,0}\)</EquationSource> <EquationSource Format="MATHML"><math> <mfenced close="]" open="["> <mfrac linethickness="0pt"> <mi>V</mi> <mrow> <mi>n</mi> <mo>,</mo> <mn>0</mn> </mrow> </mfrac> </mfenced> </math></EquationSource> </InlineEquation>.</p>

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Non-trivial t-intersecting families for the distance-regular graphs of bilinear forms

  • Mengyu Cao,
  • Benjian Lv,
  • Kaishun Wang

摘要

Let V be an \((n+\ell )\) ( n + ) -dimensional vector space over a finite field, and W a fixed \(\ell \) -dimensional subspace of V. Write \({V\brack n,0}\) V n , 0 to be the set of all n-dimensional subspaces U of V satisfying \(\dim (U\cap W)=0\) dim ( U W ) = 0 . A family \(\mathcal {F}\subseteq {V\brack n,0}\) F V n , 0 is t-intersecting if \(\dim (A\cap B)\ge t\) dim ( A B ) t for all \(A,B\in \mathcal {F}\) A , B F . A t-intersecting family \(\mathcal {F}\subseteq {V\brack n,0}\) F V n , 0 is called non-trivial if \(\dim (\cap _{F\in \mathcal {F}}F)<t\) dim ( F F F ) < t . In this paper, we describe the structure of non-trivial t-intersecting families of \({V\brack n,0}\) V n , 0 with large size. In particular, we show the structure of the non-trivial t-intersecting families with maximum size, which extends the Hilton–Milner theorem for \({V\brack n,0}\) V n , 0 .