English

On a variant of dichromatic number for digraphs with prescribed sets of arcs

Combinatorics 2023-07-13 v1

Abstract

In this paper, we consider a variant of dichromatic number on digraphs with prescribed sets of arcs. Let DD be a digraph and let Z1,Z2Z_1, Z_2 be two sets of arcs in DD. For a subdigraph HH of DD, let A(H)A(H) denote the set of all arcs of HH. Let μ(D,Z1,Z2)\mu(D, Z_1, Z_2) be the minimum number of parts in a vertex partition P\mathcal{P} of DD such that for every XPX\in \mathcal{P}, the subdigraph of DD induced by XX contains no directed cycle CC with A(C)Z1A(C)Z2|A(C)\cap Z_1|\neq |A(C)\cap Z_2|. For Z1=A(D)Z_1=A(D) and Z2=Z_2=\emptyset, μ(D,Z1,Z2)\mu(D, Z_1, Z_2) is equal to the dichromatic number of DD. We prove that for every digraph FF and every tuple (ae,be,re,qe)(a_e,b_e,r_e, q_e) of integers with qe2q_e\ge 2 and gcd(ae,qe)=gcd(be,qe)=1\gcd(a_e,q_e)=\gcd(b_e,q_e)=1 for each arc ee of FF, there exists an integer NN such that if μ(D,Z1,Z2)N\mu(D, Z_1, Z_2)\ge N, then DD contains a subdigraph isomorphic to a subdivision of FF in which each arc ee of FF is subdivided into a directed path~PeP_e such that~aeA(Pe)Z1+beA(Pe)Z2re(modqe)a_e|A(P_e)\cap Z_1|+b_e|A(P_e)\cap Z_2|\equiv {r_e}\pmod {q_e}. This generalizes a theorem of Steiner [Subdivisions with congruence constraints in digraphs of large chromatic number, arXiv:2208.06358] which corresponds to the case when (ae,be,Z1,Z2)=(1,1,A(D),)(a_e, b_e, Z_1, Z_2)=(1, 1, A(D), \emptyset).

Keywords

Cite

@article{arxiv.2307.05897,
  title  = {On a variant of dichromatic number for digraphs with prescribed sets of arcs},
  author = {O-joung Kwon and Xiaopan Lian},
  journal= {arXiv preprint arXiv:2307.05897},
  year   = {2023}
}

Comments

13 pages, 2 figures