5 research outputs found
On the choosability of -minor-free graphs
Given a graph , let us denote by and ,
respectively, the maximum chromatic number and the maximum list chromatic
number of -minor-free graphs. Hadwiger's famous coloring conjecture from
1943 states that for every . In contrast, for list
coloring it is known that
and thus, is bounded away from the conjectured value for
by at least a constant factor. The so-called -Hadwiger's
conjecture, proposed by Seymour, asks to prove that
for a given graph (which would be implied by Hadwiger's conjecture). In
this paper, we prove several new lower bounds on , thus exploring
the limits of a list coloring extension of -Hadwiger's conjecture. Our main
results are:
For every and all sufficiently large graphs we have
, where
denotes the vertex-connectivity of .
For every there exists such that
asymptotically almost every -vertex graph with edges satisfies .
The first result generalizes recent results on complete and complete
bipartite graphs and shows that the list chromatic number of -minor-free
graphs is separated from the natural lower bound by a
constant factor for all large graphs of linear connectivity. The second
result tells us that even when is a very sparse graph (with an average
degree just logarithmic in its order), can still be separated from
by a constant factor arbitrarily close to . Conceptually
these results indicate that the graphs for which is close to
are typically rather sparse.Comment: 14 page
Limits of degeneracy for colouring graphs with forbidden minors
Motivated by Hadwiger's conjecture, Seymour asked which graphs have the
property that every non-null graph with no minor has a vertex of degree
at most . We show that for every monotone graph family
with strongly sublinear separators, all sufficiently large bipartite graphs with bounded maximum degree have this property. None of the
conditions that belongs to , that is bipartite and that
has bounded maximum degree can be omitted.Comment: 22 page
Proper conflict-free list-coloring, odd minors, subdivisions, and layered treewidth
Proper conflict-free coloring is an intermediate notion between proper
coloring of a graph and proper coloring of its square. It is a proper coloring
such that for every non-isolated vertex, there exists a color appearing exactly
once in its (open) neighborhood. Typical examples of graphs with large proper
conflict-free chromatic number include graphs with large chromatic number and
bipartite graphs isomorphic to the -subdivision of graphs with large
chromatic number. In this paper, we prove two rough converse statements that
hold even in the list-coloring setting. The first is for sparse graphs: for
every graph , there exists an integer such that every graph with no
subdivision of is (properly) conflict-free -choosable. The second
applies to dense graphs: every graph with large conflict-free choice number
either contains a large complete graph as an odd minor or contains a bipartite
induced subgraph that has large conflict-free choice number. These give two
incomparable (partial) answers of a question of Caro, Petru\v{s}evski and
\v{S}krekovski. We also prove quantitatively better bounds for minor-closed
families, implying some known results about proper conflict-free coloring and
odd coloring in the literature. Moreover, we prove that every graph with
layered treewidth at most is (properly) conflict-free -choosable.
This result applies to -planar graphs, which are graphs whose coloring
problems have attracted attention recently.Comment: Hickingbotham recently independently announced a paper
(arXiv:2203.10402) proving a result similar to the ones in this paper. Please
see the notes at the end of this paper for details. v2: add results for odd
minors, which applies to graphs with unbounded degeneracy, and change the
title of the pape
Coloring Graphs with Forbidden Minors
A graph H is a minor of a graph G if H can be obtained from a subgraph of G by contracting edges. My research is motivated by the famous Hadwiger\u27s Conjecture from 1943 which states that every graph with no Kt-minor is (t β 1)-colorable. This conjecture has been proved true for t β€ 6, but remains open for all t β₯ 7. For t = 7, it is not even yet known if a graph with no K7-minor is 7-colorable. We begin by showing that every graph with no Kt-minor is (2t β 6)- colorable for t = 7, 8, 9, in the process giving a shorter and computer-free proof of the known results for t = 7, 8. We also show that this result extends to larger values of t if Mader\u27s bound for the extremal function for Kt-minors is true. Additionally, we show that any graph with no Kβ8 - minor is 9-colorable, and any graph with no K=8-minor is 8-colorable. The Kempe-chain method developed for our proofs of the above results may be of independent interest. We also use Mader\u27s H-Wege theorem to establish some sufficient conditions for a graph to contain a K8-minor. Another motivation for my research is a well-known conjecture of Erdos and Lovasz from 1968, the Double-Critical Graph Conjecture. A connected graph G is double-critical if for all edges xy β E(G), Ο(Gβxβy) = Ο(G)β2. Erdos and Lovasz conjectured that the only double-critical t-chromatic graph is the complete graph Kt. This conjecture has been show to be true for t β€ 5 and remains open for t β₯ 6. It has further been shown that any non-complete, double-critical, t-chromatic graph contains Kt as a minor for t β€ 8. We give a shorter proof of this result for t = 7, a computer-free proof for t = 8, and extend the result to show that G contains a K9-minor for all t β₯ 9. Finally, we show that the Double-Critical Graph Conjecture is true for double-critical graphs with chromatic number t β€ 8 if such graphs are claw-free