<p>For a graph <i>G</i> and an integer <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(t \ge 2\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>t</mi> <mo>≥</mo> <mn>2</mn> </mrow> </math></EquationSource> </InlineEquation>, a spanning subgraph <i>H</i> of <i>G</i> is called a <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(P_{\ge t}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mrow> <mo>≥</mo> <mi>t</mi> </mrow> </msub> </math></EquationSource> </InlineEquation>-factor of <i>G</i> if each component of <i>H</i> is a path of order at least <i>t</i>. If for any edge <InlineEquation ID="IEq6"> <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>, there is a <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(P_{\ge t}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mrow> <mo>≥</mo> <mi>t</mi> </mrow> </msub> </math></EquationSource> </InlineEquation>-factor of <i>G</i> containing <i>e</i>, then we call <i>G</i> a <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(P_{\ge t}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mrow> <mo>≥</mo> <mi>t</mi> </mrow> </msub> </math></EquationSource> </InlineEquation>-factor covered graph. Furthermore, if for any two distinct edges <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(e_1\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>e</mi> <mn>1</mn> </msub> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(e_2\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>e</mi> <mn>2</mn> </msub> </math></EquationSource> </InlineEquation> of <i>E</i>(<i>G</i>), there is a <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(P_{\ge t}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mrow> <mo>≥</mo> <mi>t</mi> </mrow> </msub> </math></EquationSource> </InlineEquation>-factor of <i>G</i> containing <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(e_1\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>e</mi> <mn>1</mn> </msub> </math></EquationSource> </InlineEquation> and excluding <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(e_2\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>e</mi> <mn>2</mn> </msub> </math></EquationSource> </InlineEquation>, then we call <i>G</i> a <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(P_{\ge t}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mrow> <mo>≥</mo> <mi>t</mi> </mrow> </msub> </math></EquationSource> </InlineEquation>-factor uniform graph. In this note, we establish some sufficient conditions related to toughness and isolated toughness for graphs (under degree constraints) to be <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(P_{\ge 3}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mrow> <mo>≥</mo> <mn>3</mn> </mrow> </msub> </math></EquationSource> </InlineEquation>-factor covered or <InlineEquation ID="IEq16"> <EquationSource Format="TEX">\(P_{\ge 3}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mrow> <mo>≥</mo> <mn>3</mn> </mrow> </msub> </math></EquationSource> </InlineEquation>-factor uniform. We also show the conditions are best possible. Our results improve some known results in the literature.</p>

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Toughness conditions for \(P_{\ge 3}\)-factors in graphs

  • Zhenyu Hong,
  • Ping Zhang,
  • Xinyue Zhou

摘要

For a graph G and an integer \(t \ge 2\) t 2 , a spanning subgraph H of G is called a \(P_{\ge t}\) P t -factor of G if each component of H is a path of order at least t. If for any edge \(e \in E(G)\) e E ( G ) , there is a \(P_{\ge t}\) P t -factor of G containing e, then we call G a \(P_{\ge t}\) P t -factor covered graph. Furthermore, if for any two distinct edges \(e_1\) e 1 and \(e_2\) e 2 of E(G), there is a \(P_{\ge t}\) P t -factor of G containing \(e_1\) e 1 and excluding \(e_2\) e 2 , then we call G a \(P_{\ge t}\) P t -factor uniform graph. In this note, we establish some sufficient conditions related to toughness and isolated toughness for graphs (under degree constraints) to be \(P_{\ge 3}\) P 3 -factor covered or \(P_{\ge 3}\) P 3 -factor uniform. We also show the conditions are best possible. Our results improve some known results in the literature.