<p>Let <i>G</i> be a graph. A hypergraph <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(H^{*}\)</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mi>H</mi> <mrow /> <mrow> <mrow /> <mo>∗</mo> </mrow> </mmultiscripts> </math></EquationSource> </InlineEquation> is called a Berge-<i>G</i> if there is a bijection <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\phi : E(G)\rightarrow E(H^{*})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>ϕ</mi> <mo>:</mo> <mi>E</mi> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> <mo stretchy="false">→</mo> <mi>E</mi> <mrow> <mo stretchy="false">(</mo> <mmultiscripts> <mi>H</mi> <mrow /> <mrow> <mrow /> <mo>∗</mo> </mrow> </mmultiscripts> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> such that <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(e\subseteq \phi (e)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>e</mi> <mo>⊆</mo> <mi>ϕ</mi> <mo stretchy="false">(</mo> <mi>e</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> for all <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(e\in E(G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>e</mi> <mo>∈</mo> <mi>E</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, where <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(E(\cdot )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>E</mi> <mo stretchy="false">(</mo> <mo>·</mo> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is the set of edges. A hypergraph <i>H</i> is said to be Berge-<i>G</i> free if <i>H</i> does not contain a Berge-<i>G</i> as its subhypergraph. Let <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(k\geqslant 3\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>k</mi> <mo>⩾</mo> <mn>3</mn> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(n\geqslant 5\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>⩾</mo> <mn>5</mn> </mrow> </math></EquationSource> </InlineEquation>. For the connected linear <i>k</i>-uniform hypergraphs on <i>n</i> vertices without the Berge-<InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(\{C_3,K_{2,3}\}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">{</mo> <msub> <mi>C</mi> <mn>3</mn> </msub> <mo>,</mo> <msub> <mi>K</mi> <mrow> <mn>2</mn> <mo>,</mo> <mn>3</mn> </mrow> </msub> <mo stretchy="false">}</mo> </mrow> </math></EquationSource> </InlineEquation> or the Berge-<InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\{F_2,K_{2,3}\}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">{</mo> <msub> <mi>F</mi> <mn>2</mn> </msub> <mo>,</mo> <msub> <mi>K</mi> <mrow> <mn>2</mn> <mo>,</mo> <mn>3</mn> </mrow> </msub> <mo stretchy="false">}</mo> </mrow> </math></EquationSource> </InlineEquation>, the upper bounds of their spectral radii regarding to <i>n</i> and <i>k</i> are derived, where <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(C_3\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>C</mi> <mn>3</mn> </msub> </math></EquationSource> </InlineEquation> is a cycle of length 3, <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(F_2\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>F</mi> <mn>2</mn> </msub> </math></EquationSource> </InlineEquation> a graph obtained from two disjoint <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(C_3\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>C</mi> <mn>3</mn> </msub> </math></EquationSource> </InlineEquation> by identifying a vertex of <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(C_3\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>C</mi> <mn>3</mn> </msub> </math></EquationSource> </InlineEquation> with a vertex of another <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(C_3\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>C</mi> <mn>3</mn> </msub> </math></EquationSource> </InlineEquation>, and <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(K_{2,3}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>K</mi> <mrow> <mn>2</mn> <mo>,</mo> <mn>3</mn> </mrow> </msub> </math></EquationSource> </InlineEquation> a complete bipartite graph with two parts of sizes 2 and 3.</p>

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The extremal problems for the spectral radius of hypergraphs without the Berge-graphs

  • Hao-Ran Zhang,
  • Wen-Huan Wang,
  • Huan-Lei Shi

摘要

Let G be a graph. A hypergraph \(H^{*}\) H is called a Berge-G if there is a bijection \(\phi : E(G)\rightarrow E(H^{*})\) ϕ : E ( G ) E ( H ) such that \(e\subseteq \phi (e)\) e ϕ ( e ) for all \(e\in E(G)\) e E ( G ) , where \(E(\cdot )\) E ( · ) is the set of edges. A hypergraph H is said to be Berge-G free if H does not contain a Berge-G as its subhypergraph. Let \(k\geqslant 3\) k 3 and \(n\geqslant 5\) n 5 . For the connected linear k-uniform hypergraphs on n vertices without the Berge- \(\{C_3,K_{2,3}\}\) { C 3 , K 2 , 3 } or the Berge- \(\{F_2,K_{2,3}\}\) { F 2 , K 2 , 3 } , the upper bounds of their spectral radii regarding to n and k are derived, where \(C_3\) C 3 is a cycle of length 3, \(F_2\) F 2 a graph obtained from two disjoint \(C_3\) C 3 by identifying a vertex of \(C_3\) C 3 with a vertex of another \(C_3\) C 3 , and \(K_{2,3}\) K 2 , 3 a complete bipartite graph with two parts of sizes 2 and 3.