<p>Given a graph <i>G</i>, the <i>k</i>-coloring graph <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\mathcal {C}_k(G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="script">C</mi> <mi>k</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is constructed by selecting proper <i>k</i>-colorings of <i>G</i> as vertices, with an edge between two colorings if they differ in the color of exactly one vertex. The number of vertices in <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\mathcal {C}_k(G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="script">C</mi> <mi>k</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is the famous chromatic polynomial of <i>G</i>. Asgarli, Krehbiel, Levinson and Russell showed that for any subgraph <i>H</i>, the number of induced copies of <i>H</i> in <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\mathcal {C}_k(G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="script">C</mi> <mi>k</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is a polynomial function in <i>k</i>. Hogan, Scott, Tamitegama, and Tan found a shorter proof for polynomiality of these chromatic <i>H</i>-polynomials. In this paper, we provide a method of constructing these polynomials explicitly in terms of chromatic polynomials of <i>shadow graphs</i>. We illustrate the practicality of our formulas by computing an explicit formula for <i>H</i>-polynomial for trees when <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(H=Q_d\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>H</mi> <mo>=</mo> <msub> <mi>Q</mi> <mi>d</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> is an arbitrary hypercube, a task which does not seem approachable from previous methods. The coefficients of the resulting polynomials feature <i>generalized degree sequences</i> introduced by Crew. In the special case when <i>H</i> is the complete graph on 2 vertices, the corresponding polynomial is dubbed the <i>chromatic pairs polynomial</i>. We present a pair of graphs <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(G_1\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>G</mi> <mn>1</mn> </msub> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(G_2\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>G</mi> <mn>2</mn> </msub> </math></EquationSource> </InlineEquation> sharing the same chromatic pairs polynomial but different chromatic polynomials, disproving a conjecture raised by Asgarli, Krehbiel, Levinson and Russell.</p>

错误:搜索内容不能为空,请输入英文关键词
错误:关键词超出字数限制,请精简
高级检索

Counting Subgraphs of Coloring Graphs Using Shadow Graphs

  • Simon MacLean

摘要

Given a graph G, the k-coloring graph \(\mathcal {C}_k(G)\) C k ( G ) is constructed by selecting proper k-colorings of G as vertices, with an edge between two colorings if they differ in the color of exactly one vertex. The number of vertices in \(\mathcal {C}_k(G)\) C k ( G ) is the famous chromatic polynomial of G. Asgarli, Krehbiel, Levinson and Russell showed that for any subgraph H, the number of induced copies of H in \(\mathcal {C}_k(G)\) C k ( G ) is a polynomial function in k. Hogan, Scott, Tamitegama, and Tan found a shorter proof for polynomiality of these chromatic H-polynomials. In this paper, we provide a method of constructing these polynomials explicitly in terms of chromatic polynomials of shadow graphs. We illustrate the practicality of our formulas by computing an explicit formula for H-polynomial for trees when \(H=Q_d\) H = Q d is an arbitrary hypercube, a task which does not seem approachable from previous methods. The coefficients of the resulting polynomials feature generalized degree sequences introduced by Crew. In the special case when H is the complete graph on 2 vertices, the corresponding polynomial is dubbed the chromatic pairs polynomial. We present a pair of graphs \(G_1\) G 1 and \(G_2\) G 2 sharing the same chromatic pairs polynomial but different chromatic polynomials, disproving a conjecture raised by Asgarli, Krehbiel, Levinson and Russell.