English

Borodin-Kostochka conjecture and Partitioning a graph into classes with no clique of specified size

Combinatorics 2023-11-16 v1

Abstract

For a given graph HH and the graphical properties P1,P2,,PkP_1, P_2,\ldots,P_k, a graph HH is said to be (V1,V2,,Vk)(V_1, V_2,\ldots,V_k)-partitionable if there exists a partition of V(H)V(H) into kk-sets V1,V2,VkV_1, V_2\ldots,V_k, such that for each i[k]i\in[k], the subgraph induced by ViV_i has the property PiP_i. In 19791979, Bollob\'{a}s and Manvel showed that for a graph HH with maximum degree Δ(H)3\Delta(H)\geq 3 and clique number ω(H)Δ(H)\omega(H)\leq \Delta(H), if Δ(H)=p+q\Delta(H)= p+q, then there exists a (V1,V2)(V_1,V_2)-partition of V(H)V(H), such that Δ(H[V1])p\Delta(H[V_1])\leq p, Δ(H[V2])q\Delta(H[V_2])\leq q, H[V1]H[V_1] is (p1)(p-1)-degenerate, and H[V2]H[V_2] is (q1)(q-1)-degenerate. Assume that p1p2pk2p_1\geq p_2\geq\cdots\geq p_k\geq 2 are kk positive integers and i=1kpi=Δ(H)1+k\sum_{i=1}^k p_i=\Delta(H)-1+k. Assume that for each i[k]i\in[k] the properties PiP_i means that ω(H[Vi])pi1\omega(H[V_i])\leq p_i-1. Is HH a (V1,,Vk)(V_1,\ldots,V_k)-partitionable graph? In 1977, Borodin and Kostochka conjectured that any graph HH with maximum degree Δ(H)9\Delta(H)\geq 9 and without KΔ(H)K_{\Delta(H)} as a subgraph, has chromatic number at most Δ(H)1\Delta(H)-1. Reed proved that the conjecture holds whenever Δ(G)1014 \Delta(G) \geq 10^{14} . When p1=2p_1=2 and Δ(H)9\Delta(H)\geq 9, the above question is the Borodin and Kostochka conjecture. Therefore, when all pip_is are equal to 22 and Δ(H)8\Delta(H)\leq 8, the answer to the above question is negative. Let HH is a graph with maximum degree Δ\Delta, and clique number ω(H)\omega(H), where ω(H)Δ1\omega(H)\leq \Delta-1. In this article, we intend to study this question when k2k\geq 2 and Δ13\Delta\geq 13. In particular as an analogue of the Borodin-Kostochka conjecture, for the case that Δ13\Delta\geq 13 and pi2p_i\geq 2 we prove that the above question is true.

Keywords

Cite

@article{arxiv.2311.08772,
  title  = {Borodin-Kostochka conjecture and Partitioning a graph into classes with no clique of specified size},
  author = {Yaser Rowshan},
  journal= {arXiv preprint arXiv:2311.08772},
  year   = {2023}
}