16 research outputs found
Infinite families of -vertex-critical (, )-free graphs
A graph is -vertex-critical if but for all . We construct a new infinite families of -vertex-critical
-free graphs for all . Our construction generalizes known
constructions for -vertex-critical -free graphs and -vertex-critical
-free graphs and is in contrast to the fact that there are only finitely
many -vertex-critical -free graphs. In fact, our construction is
actually even more well-structured, being -free
Exhaustive generation of -critical -free graphs
We describe an algorithm for generating all -critical -free
graphs, based on a method of Ho\`{a}ng et al. Using this algorithm, we prove
that there are only finitely many -critical -free graphs, for
both and . We also show that there are only finitely many
-critical graphs -free graphs. For each case of these cases we
also give the complete lists of critical graphs and vertex-critical graphs.
These results generalize previous work by Hell and Huang, and yield certifying
algorithms for the -colorability problem in the respective classes.
Moreover, we prove that for every , the class of 4-critical planar
-free graphs is finite. We also determine all 27 4-critical planar
-free graphs.
We also prove that every -free graph of girth at least five is
3-colorable, and determine the smallest 4-chromatic -free graph of
girth five. Moreover, we show that every -free graph of girth at least
six and every -free graph of girth at least seven is 3-colorable. This
strengthens results of Golovach et al.Comment: 17 pages, improved girth results. arXiv admin note: text overlap with
arXiv:1504.0697
A refinement on the structure of vertex-critical (, gem)-free graphs
We give a new, stronger proof that there are only finitely many
-vertex-critical (,~gem)-free graphs for all . Our proof further
refines the structure of these graphs and allows for the implementation of a
simple exhaustive computer search to completely list all - and
-vertex-critical , gem)-free graphs. Our results imply the existence
of polynomial-time certifying algorithms to decide the -colourability of
, gem)-free graphs for all where the certificate is either a
-colouring or a -vertex-critical induced subgraph. Our complete lists
for allow for the implementation of these algorithms for all
Critical -Free Graphs
Given two graphs and , a graph is -free if it contains
no induced subgraph isomorphic to nor . Let be the path on
vertices. A dart is the graph obtained from a diamond by adding a new vertex
and making it adjacent to exactly one vertex with degree 3 in the diamond.
In this paper, we show that there are finitely many -vertex-critical
-free graphs for To prove these results, we use induction
on and perform a careful structural analysis via Strong Perfect Graph
Theorem combined with the pigeonhole principle based on the properties of
vertex-critical graphs. Moreover, for we characterize all
-vertex-critical -free graphs using a computer generation
algorithm. Our results imply the existence of a polynomial-time certifying
algorithm to decide the -colorability of -free graphs for where the certificate is either a -coloring or a -vertex-critical
induced subgraph.Comment: arXiv admin note: text overlap with arXiv:2211.0417
Critical (,bull)-free graphs
Given two graphs and , a graph is -free if it contains
no induced subgraph isomorphic to or . Let and be the
path and the cycle on vertices, respectively. A bull is the graph obtained
from a triangle with two disjoint pendant edges. In this paper, we show that
there are finitely many 5-vertex-critical (,bull)-free graphs.Comment: 21 page