A counterexample to a conjecture about triangle-free induced subgraphs of graphs with large chromatic number

نویسندگان

چکیده

We prove that for every $n$, there is a graph $G$ with $\chi(G) \geq n$ and $\omega(G) \leq 3$ such induced subgraph $H$ of $\omega(H) 2$ satisfies $\chi(H) 4$. This disproves well-known conjecture. Our construction digraph bounded clique number, large dichromatic no directed cycles odd length at least 5.

برای دانلود رایگان متن کامل این مقاله و بیش از 32 میلیون مقاله دیگر ابتدا ثبت نام کنید

اگر عضو سایت هستید لطفا وارد حساب کاربری خود شوید

منابع مشابه

Triangle-free subgraphs with large fractional chromatic number

It is well known that for any k and g, there is a graph with chromatic number at least k and girth at least g. In 1970’s, Erdős and Hajnal conjectured that for any numbers k and g, there exists a number f(k, g), such that every graph with chromatic number at least f(k, g) contains a subgraph with chromatic number at least k and girth at least g. In 1978, Rödl proved the case for g = 4 and arbit...

متن کامل

Induced subgraphs of graphs with large chromatic number. XI. Orientations

Fix an oriented graph H, and let G be a graph with bounded clique number and very large chromatic number. If we somehow orient its edges, must there be an induced subdigraph isomorphic to H? Kierstead and Rödl [12] raised this question for two specific kinds of digraph H: the three-edge path, with the first and last edges both directed towards the interior; and stars (with many edges directed o...

متن کامل

Triangle-free geometric intersection graphs with large chromatic number

Several classical constructions illustrate the fact that the chromatic number of a graph may be arbitrarily large compared to its clique number. However, until very recently no such construction was known for intersection graphs of geometric objects in the plane. We provide a general construction that for any arc-connected compact set X in R that is not an axis-aligned rectangle and for any pos...

متن کامل

Induced subgraphs of graphs with large chromatic number. VII. Gyárfás’ complementation conjecture

A class of graphs is χ-bounded if there is a function f such that χ(G) ≤ f(ω(G)) for every induced subgraph G of every graph in the class, where χ, ω denote the chromatic number and clique number of G respectively. In 1987, Gyárfás conjectured that for every c, if C is a class of graphs such that χ(G) ≤ ω(G) + c for every induced subgraph G of every graph in the class, then the class of complem...

متن کامل

Cycles in triangle-free graphs of large chromatic number

More than twenty years ago Erdős conjectured [4] that a triangle-free graph G of chromatic number k ≥ k0(ε) contains cycles of at least k2−ε different lengths as k →∞. In this paper, we prove the stronger fact that every triangle-free graph G of chromatic number k ≥ k0(ε) contains cycles of 1 64 (1− ε)k 2 log k4 consecutive lengths, and a cycle of length at least 14 (1− ε)k 2 log k. As there ex...

متن کامل

ذخیره در منابع من


  با ذخیره ی این منبع در منابع من، دسترسی به آن را برای استفاده های بعدی آسان تر کنید

ژورنال

عنوان ژورنال: Journal of Combinatorial Theory, Series B

سال: 2023

ISSN: ['0095-8956', '1096-0902']

DOI: https://doi.org/10.1016/j.jctb.2022.09.001