Manu Basavaraju, L. Sunil Chandran, Mathew C. Francis, Karthik Murali
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{"title":"Gyárfás-Sumner 猜想的变体:定向树和彩虹路径","authors":"Manu Basavaraju, L. Sunil Chandran, Mathew C. Francis, Karthik Murali","doi":"10.1002/jgt.23171","DOIUrl":null,"url":null,"abstract":"<p>Given a finite family <span></span><math>\n \n <mrow>\n <mi>ℱ</mi>\n </mrow></math> of graphs, we say that a graph <span></span><math>\n \n <mrow>\n <mi>G</mi>\n </mrow></math> is “<span></span><math>\n \n <mrow>\n <mi>ℱ</mi>\n </mrow></math>-free” if <span></span><math>\n \n <mrow>\n <mi>G</mi>\n </mrow></math> does not contain any graph in <span></span><math>\n \n <mrow>\n <mi>ℱ</mi>\n </mrow></math> as a subgraph. We abbreviate <span></span><math>\n \n <mrow>\n <mi>ℱ</mi>\n </mrow></math>-free to just “<span></span><math>\n \n <mrow>\n <mi>F</mi>\n </mrow></math>-free” when <span></span><math>\n \n <mrow>\n <mi>ℱ</mi>\n \n <mo>=</mo>\n \n <mrow>\n <mo>{</mo>\n \n <mi>F</mi>\n \n <mo>}</mo>\n </mrow>\n </mrow></math>. A vertex-colored graph <span></span><math>\n \n <mrow>\n <mi>H</mi>\n </mrow></math> is called “rainbow” if no two vertices of <span></span><math>\n \n <mrow>\n <mi>H</mi>\n </mrow></math> have the same color. Given an integer <span></span><math>\n \n <mrow>\n <mi>s</mi>\n </mrow></math> and a finite family of graphs <span></span><math>\n \n <mrow>\n <mi>ℱ</mi>\n </mrow></math>, let <span></span><math>\n \n <mrow>\n <mi>ℓ</mi>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>,</mo>\n \n <mi>ℱ</mi>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n </mrow></math> denote the smallest integer such that any properly vertex-colored <span></span><math>\n \n <mrow>\n <mi>ℱ</mi>\n </mrow></math>-free graph <span></span><math>\n \n <mrow>\n <mi>G</mi>\n </mrow></math> having <span></span><math>\n \n <mrow>\n <mi>χ</mi>\n \n <mrow>\n <mo>(</mo>\n \n <mi>G</mi>\n \n <mo>)</mo>\n </mrow>\n \n <mo>≥</mo>\n \n <mi>ℓ</mi>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>,</mo>\n \n <mi>ℱ</mi>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n </mrow></math> contains an induced rainbow path on <span></span><math>\n \n <mrow>\n <mi>s</mi>\n </mrow></math> vertices. Scott and Seymour showed that <span></span><math>\n \n <mrow>\n <mi>ℓ</mi>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>,</mo>\n \n <mi>K</mi>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n </mrow></math> exists for every complete graph <span></span><math>\n \n <mrow>\n <mi>K</mi>\n </mrow></math>. A conjecture of N. R. Aravind states that <span></span><math>\n \n <mrow>\n <mi>ℓ</mi>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>,</mo>\n \n <msub>\n <mi>C</mi>\n \n <mn>3</mn>\n </msub>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mo>=</mo>\n \n <mi>s</mi>\n </mrow></math>. The upper bound on <span></span><math>\n \n <mrow>\n <mi>ℓ</mi>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>,</mo>\n \n <msub>\n <mi>C</mi>\n \n <mn>3</mn>\n </msub>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n </mrow></math> that can be obtained using the methods of Scott and Seymour setting <span></span><math>\n \n <mrow>\n <mi>K</mi>\n \n <mo>=</mo>\n \n <msub>\n <mi>C</mi>\n \n <mn>3</mn>\n </msub>\n </mrow></math> are, however, super-exponential. Gyárfás and Sárközy showed that <span></span><math>\n \n <mrow>\n <mi>ℓ</mi>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>,</mo>\n \n <mrow>\n <mo>{</mo>\n \n <mrow>\n <msub>\n <mi>C</mi>\n \n <mn>3</mn>\n </msub>\n \n <mo>,</mo>\n \n <msub>\n <mi>C</mi>\n \n <mn>4</mn>\n </msub>\n </mrow>\n \n <mo>}</mo>\n </mrow>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mo>=</mo>\n \n <mi>O</mi>\n \n <mrow>\n <mo>(</mo>\n \n <msup>\n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mn>2</mn>\n \n <mi>s</mi>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mrow>\n <mn>2</mn>\n \n <mi>s</mi>\n </mrow>\n </msup>\n \n <mo>)</mo>\n </mrow>\n </mrow></math>. For <span></span><math>\n \n <mrow>\n <mi>r</mi>\n \n <mo>≥</mo>\n \n <mn>2</mn>\n </mrow></math>, we show that <span></span><math>\n \n <mrow>\n <mi>ℓ</mi>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>,</mo>\n \n <msub>\n <mi>K</mi>\n \n <mrow>\n <mn>2</mn>\n \n <mo>,</mo>\n \n <mi>r</mi>\n </mrow>\n </msub>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mo>≤</mo>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>r</mi>\n \n <mo>−</mo>\n \n <mn>1</mn>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>−</mo>\n \n <mn>1</mn>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>−</mo>\n \n <mn>2</mn>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mo>∕</mo>\n \n <mn>2</mn>\n \n <mo>+</mo>\n \n <mi>s</mi>\n </mrow></math> and therefore, <span></span><math>\n \n <mrow>\n <mi>ℓ</mi>\n \n <mrow>\n <mo>(</mo>\n \n <mi>s</mi>\n \n <mo>,</mo>\n \n <msub>\n <mi>C</mi>\n \n <mn>4</mn>\n </msub>\n \n <mo>)</mo>\n </mrow>\n <mspace></mspace>\n \n <mo>≤</mo>\n <mspace></mspace>\n \n <mfrac>\n <mrow>\n <msup>\n <mi>s</mi>\n \n <mn>2</mn>\n </msup>\n \n <mo>−</mo>\n \n <mi>s</mi>\n \n <mo>+</mo>\n \n <mn>2</mn>\n </mrow>\n \n <mn>2</mn>\n </mfrac>\n </mrow></math>. This significantly improves Gyárfás and Sárközy's bound and also covers a bigger class of graphs. We adapt our proof to achieve much stronger upper bounds for graphs of higher girth: we prove that <span></span><math>\n \n <mrow>\n <mi>ℓ</mi>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>,</mo>\n \n <mrow>\n <mo>{</mo>\n \n <mrow>\n <msub>\n <mi>C</mi>\n \n <mn>3</mn>\n </msub>\n \n <mo>,</mo>\n \n <msub>\n <mi>C</mi>\n \n <mn>4</mn>\n </msub>\n \n <mo>,</mo>\n \n <mo>…</mo>\n \n <mo>,</mo>\n \n <msub>\n <mi>C</mi>\n \n <mrow>\n <mi>g</mi>\n \n <mo>−</mo>\n \n <mn>1</mn>\n </mrow>\n </msub>\n </mrow>\n \n <mo>}</mo>\n </mrow>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mo>≤</mo>\n \n <msup>\n <mi>s</mi>\n \n <mrow>\n <mn>1</mn>\n \n <mo>+</mo>\n \n <mfrac>\n <mn>4</mn>\n \n <mrow>\n <mi>g</mi>\n \n <mo>−</mo>\n \n <mn>4</mn>\n </mrow>\n </mfrac>\n </mrow>\n </msup>\n </mrow></math>, where <span></span><math>\n \n <mrow>\n <mi>g</mi>\n \n <mo>≥</mo>\n \n <mn>5</mn>\n </mrow></math>. Moreover, in each case, our results imply the existence of at least <span></span><math>\n \n <mrow>\n <mi>s</mi>\n \n <mo>!</mo>\n \n <mo>∕</mo>\n \n <mn>2</mn>\n </mrow></math> distinct induced rainbow paths on <span></span><math>\n \n <mrow>\n <mi>s</mi>\n </mrow></math> vertices. Along the way, we obtain some new results on an oriented variant of the Gyárfás–Sumner conjecture. For <span></span><math>\n \n <mrow>\n <mi>r</mi>\n \n <mo>≥</mo>\n \n <mn>2</mn>\n </mrow></math>, let <span></span><math>\n \n <mrow>\n <msub>\n <mi>ℬ</mi>\n \n <mi>r</mi>\n </msub>\n </mrow></math> denote the orientations of <span></span><math>\n \n <mrow>\n <msub>\n <mi>K</mi>\n \n <mrow>\n <mn>2</mn>\n \n <mo>,</mo>\n \n <mi>r</mi>\n </mrow>\n </msub>\n </mrow></math> in which one vertex has out-degree or in-degree <span></span><math>\n \n <mrow>\n <mi>r</mi>\n </mrow></math>. We show that every <span></span><math>\n \n <mrow>\n <msub>\n <mi>ℬ</mi>\n \n <mi>r</mi>\n </msub>\n </mrow></math>-free oriented graph having a chromatic number at least <span></span><math>\n \n <mrow>\n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>r</mi>\n \n <mo>−</mo>\n \n <mn>1</mn>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>−</mo>\n \n <mn>1</mn>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mrow>\n <mo>(</mo>\n \n <mrow>\n <mi>s</mi>\n \n <mo>−</mo>\n \n <mn>2</mn>\n </mrow>\n \n <mo>)</mo>\n </mrow>\n \n <mo>+</mo>\n \n <mn>2</mn>\n \n <mi>s</mi>\n \n <mo>+</mo>\n \n <mn>1</mn>\n </mrow></math> and every bikernel-perfect oriented graph with girth <span></span><math>\n \n <mrow>\n <mi>g</mi>\n \n <mo>≥</mo>\n \n <mn>5</mn>\n </mrow></math> having a chromatic number at least <span></span><math>\n \n <mrow>\n <mn>2</mn>\n \n <msup>\n <mi>s</mi>\n \n <mrow>\n <mn>1</mn>\n \n <mo>+</mo>\n \n <mfrac>\n <mn>4</mn>\n \n <mrow>\n <mi>g</mi>\n \n <mo>−</mo>\n \n <mn>4</mn>\n </mrow>\n </mfrac>\n </mrow>\n </msup>\n </mrow></math> contains every oriented tree on at most <span></span><math>\n \n <mrow>\n <mi>s</mi>\n </mrow></math> vertices as an induced subgraph.</p>","PeriodicalId":16014,"journal":{"name":"Journal of Graph Theory","volume":"108 1","pages":"136-161"},"PeriodicalIF":0.9000,"publicationDate":"2024-09-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Variants of the Gyárfás–Sumner conjecture: Oriented trees and rainbow paths\",\"authors\":\"Manu Basavaraju, L. Sunil Chandran, Mathew C. Francis, Karthik Murali\",\"doi\":\"10.1002/jgt.23171\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<p>Given a finite family <span></span><math>\\n \\n <mrow>\\n <mi>ℱ</mi>\\n </mrow></math> of graphs, we say that a graph <span></span><math>\\n \\n <mrow>\\n <mi>G</mi>\\n </mrow></math> is “<span></span><math>\\n \\n <mrow>\\n <mi>ℱ</mi>\\n </mrow></math>-free” if <span></span><math>\\n \\n <mrow>\\n <mi>G</mi>\\n </mrow></math> does not contain any graph in <span></span><math>\\n \\n <mrow>\\n <mi>ℱ</mi>\\n </mrow></math> as a subgraph. We abbreviate <span></span><math>\\n \\n <mrow>\\n <mi>ℱ</mi>\\n </mrow></math>-free to just “<span></span><math>\\n \\n <mrow>\\n <mi>F</mi>\\n </mrow></math>-free” when <span></span><math>\\n \\n <mrow>\\n <mi>ℱ</mi>\\n \\n <mo>=</mo>\\n \\n <mrow>\\n <mo>{</mo>\\n \\n <mi>F</mi>\\n \\n <mo>}</mo>\\n </mrow>\\n </mrow></math>. A vertex-colored graph <span></span><math>\\n \\n <mrow>\\n <mi>H</mi>\\n </mrow></math> is called “rainbow” if no two vertices of <span></span><math>\\n \\n <mrow>\\n <mi>H</mi>\\n </mrow></math> have the same color. Given an integer <span></span><math>\\n \\n <mrow>\\n <mi>s</mi>\\n </mrow></math> and a finite family of graphs <span></span><math>\\n \\n <mrow>\\n <mi>ℱ</mi>\\n </mrow></math>, let <span></span><math>\\n \\n <mrow>\\n <mi>ℓ</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>,</mo>\\n \\n <mi>ℱ</mi>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n </mrow></math> denote the smallest integer such that any properly vertex-colored <span></span><math>\\n \\n <mrow>\\n <mi>ℱ</mi>\\n </mrow></math>-free graph <span></span><math>\\n \\n <mrow>\\n <mi>G</mi>\\n </mrow></math> having <span></span><math>\\n \\n <mrow>\\n <mi>χ</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mi>G</mi>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mo>≥</mo>\\n \\n <mi>ℓ</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>,</mo>\\n \\n <mi>ℱ</mi>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n </mrow></math> contains an induced rainbow path on <span></span><math>\\n \\n <mrow>\\n <mi>s</mi>\\n </mrow></math> vertices. Scott and Seymour showed that <span></span><math>\\n \\n <mrow>\\n <mi>ℓ</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>,</mo>\\n \\n <mi>K</mi>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n </mrow></math> exists for every complete graph <span></span><math>\\n \\n <mrow>\\n <mi>K</mi>\\n </mrow></math>. A conjecture of N. R. Aravind states that <span></span><math>\\n \\n <mrow>\\n <mi>ℓ</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>,</mo>\\n \\n <msub>\\n <mi>C</mi>\\n \\n <mn>3</mn>\\n </msub>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mo>=</mo>\\n \\n <mi>s</mi>\\n </mrow></math>. The upper bound on <span></span><math>\\n \\n <mrow>\\n <mi>ℓ</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>,</mo>\\n \\n <msub>\\n <mi>C</mi>\\n \\n <mn>3</mn>\\n </msub>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n </mrow></math> that can be obtained using the methods of Scott and Seymour setting <span></span><math>\\n \\n <mrow>\\n <mi>K</mi>\\n \\n <mo>=</mo>\\n \\n <msub>\\n <mi>C</mi>\\n \\n <mn>3</mn>\\n </msub>\\n </mrow></math> are, however, super-exponential. Gyárfás and Sárközy showed that <span></span><math>\\n \\n <mrow>\\n <mi>ℓ</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>,</mo>\\n \\n <mrow>\\n <mo>{</mo>\\n \\n <mrow>\\n <msub>\\n <mi>C</mi>\\n \\n <mn>3</mn>\\n </msub>\\n \\n <mo>,</mo>\\n \\n <msub>\\n <mi>C</mi>\\n \\n <mn>4</mn>\\n </msub>\\n </mrow>\\n \\n <mo>}</mo>\\n </mrow>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mo>=</mo>\\n \\n <mi>O</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <msup>\\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mn>2</mn>\\n \\n <mi>s</mi>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mrow>\\n <mn>2</mn>\\n \\n <mi>s</mi>\\n </mrow>\\n </msup>\\n \\n <mo>)</mo>\\n </mrow>\\n </mrow></math>. For <span></span><math>\\n \\n <mrow>\\n <mi>r</mi>\\n \\n <mo>≥</mo>\\n \\n <mn>2</mn>\\n </mrow></math>, we show that <span></span><math>\\n \\n <mrow>\\n <mi>ℓ</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>,</mo>\\n \\n <msub>\\n <mi>K</mi>\\n \\n <mrow>\\n <mn>2</mn>\\n \\n <mo>,</mo>\\n \\n <mi>r</mi>\\n </mrow>\\n </msub>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mo>≤</mo>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>r</mi>\\n \\n <mo>−</mo>\\n \\n <mn>1</mn>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>−</mo>\\n \\n <mn>1</mn>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>−</mo>\\n \\n <mn>2</mn>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mo>∕</mo>\\n \\n <mn>2</mn>\\n \\n <mo>+</mo>\\n \\n <mi>s</mi>\\n </mrow></math> and therefore, <span></span><math>\\n \\n <mrow>\\n <mi>ℓ</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mi>s</mi>\\n \\n <mo>,</mo>\\n \\n <msub>\\n <mi>C</mi>\\n \\n <mn>4</mn>\\n </msub>\\n \\n <mo>)</mo>\\n </mrow>\\n <mspace></mspace>\\n \\n <mo>≤</mo>\\n <mspace></mspace>\\n \\n <mfrac>\\n <mrow>\\n <msup>\\n <mi>s</mi>\\n \\n <mn>2</mn>\\n </msup>\\n \\n <mo>−</mo>\\n \\n <mi>s</mi>\\n \\n <mo>+</mo>\\n \\n <mn>2</mn>\\n </mrow>\\n \\n <mn>2</mn>\\n </mfrac>\\n </mrow></math>. This significantly improves Gyárfás and Sárközy's bound and also covers a bigger class of graphs. We adapt our proof to achieve much stronger upper bounds for graphs of higher girth: we prove that <span></span><math>\\n \\n <mrow>\\n <mi>ℓ</mi>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>,</mo>\\n \\n <mrow>\\n <mo>{</mo>\\n \\n <mrow>\\n <msub>\\n <mi>C</mi>\\n \\n <mn>3</mn>\\n </msub>\\n \\n <mo>,</mo>\\n \\n <msub>\\n <mi>C</mi>\\n \\n <mn>4</mn>\\n </msub>\\n \\n <mo>,</mo>\\n \\n <mo>…</mo>\\n \\n <mo>,</mo>\\n \\n <msub>\\n <mi>C</mi>\\n \\n <mrow>\\n <mi>g</mi>\\n \\n <mo>−</mo>\\n \\n <mn>1</mn>\\n </mrow>\\n </msub>\\n </mrow>\\n \\n <mo>}</mo>\\n </mrow>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mo>≤</mo>\\n \\n <msup>\\n <mi>s</mi>\\n \\n <mrow>\\n <mn>1</mn>\\n \\n <mo>+</mo>\\n \\n <mfrac>\\n <mn>4</mn>\\n \\n <mrow>\\n <mi>g</mi>\\n \\n <mo>−</mo>\\n \\n <mn>4</mn>\\n </mrow>\\n </mfrac>\\n </mrow>\\n </msup>\\n </mrow></math>, where <span></span><math>\\n \\n <mrow>\\n <mi>g</mi>\\n \\n <mo>≥</mo>\\n \\n <mn>5</mn>\\n </mrow></math>. Moreover, in each case, our results imply the existence of at least <span></span><math>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>!</mo>\\n \\n <mo>∕</mo>\\n \\n <mn>2</mn>\\n </mrow></math> distinct induced rainbow paths on <span></span><math>\\n \\n <mrow>\\n <mi>s</mi>\\n </mrow></math> vertices. Along the way, we obtain some new results on an oriented variant of the Gyárfás–Sumner conjecture. For <span></span><math>\\n \\n <mrow>\\n <mi>r</mi>\\n \\n <mo>≥</mo>\\n \\n <mn>2</mn>\\n </mrow></math>, let <span></span><math>\\n \\n <mrow>\\n <msub>\\n <mi>ℬ</mi>\\n \\n <mi>r</mi>\\n </msub>\\n </mrow></math> denote the orientations of <span></span><math>\\n \\n <mrow>\\n <msub>\\n <mi>K</mi>\\n \\n <mrow>\\n <mn>2</mn>\\n \\n <mo>,</mo>\\n \\n <mi>r</mi>\\n </mrow>\\n </msub>\\n </mrow></math> in which one vertex has out-degree or in-degree <span></span><math>\\n \\n <mrow>\\n <mi>r</mi>\\n </mrow></math>. We show that every <span></span><math>\\n \\n <mrow>\\n <msub>\\n <mi>ℬ</mi>\\n \\n <mi>r</mi>\\n </msub>\\n </mrow></math>-free oriented graph having a chromatic number at least <span></span><math>\\n \\n <mrow>\\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>r</mi>\\n \\n <mo>−</mo>\\n \\n <mn>1</mn>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>−</mo>\\n \\n <mn>1</mn>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mrow>\\n <mo>(</mo>\\n \\n <mrow>\\n <mi>s</mi>\\n \\n <mo>−</mo>\\n \\n <mn>2</mn>\\n </mrow>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mo>+</mo>\\n \\n <mn>2</mn>\\n \\n <mi>s</mi>\\n \\n <mo>+</mo>\\n \\n <mn>1</mn>\\n </mrow></math> and every bikernel-perfect oriented graph with girth <span></span><math>\\n \\n <mrow>\\n <mi>g</mi>\\n \\n <mo>≥</mo>\\n \\n <mn>5</mn>\\n </mrow></math> having a chromatic number at least <span></span><math>\\n \\n <mrow>\\n <mn>2</mn>\\n \\n <msup>\\n <mi>s</mi>\\n \\n <mrow>\\n <mn>1</mn>\\n \\n <mo>+</mo>\\n \\n <mfrac>\\n <mn>4</mn>\\n \\n <mrow>\\n <mi>g</mi>\\n \\n <mo>−</mo>\\n \\n <mn>4</mn>\\n </mrow>\\n </mfrac>\\n </mrow>\\n </msup>\\n </mrow></math> contains every oriented tree on at most <span></span><math>\\n \\n <mrow>\\n <mi>s</mi>\\n </mrow></math> vertices as an induced subgraph.</p>\",\"PeriodicalId\":16014,\"journal\":{\"name\":\"Journal of Graph Theory\",\"volume\":\"108 1\",\"pages\":\"136-161\"},\"PeriodicalIF\":0.9000,\"publicationDate\":\"2024-09-10\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Journal of Graph Theory\",\"FirstCategoryId\":\"100\",\"ListUrlMain\":\"https://onlinelibrary.wiley.com/doi/10.1002/jgt.23171\",\"RegionNum\":3,\"RegionCategory\":\"数学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q2\",\"JCRName\":\"MATHEMATICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of Graph Theory","FirstCategoryId":"100","ListUrlMain":"https://onlinelibrary.wiley.com/doi/10.1002/jgt.23171","RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"MATHEMATICS","Score":null,"Total":0}
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