9 research outputs found
Equitable partition of graphs into induced forests
An equitable partition of a graph is a partition of the vertex-set of
such that the sizes of any two parts differ by at most one. We show that every
graph with an acyclic coloring with at most colors can be equitably
partitioned into induced forests. We also prove that for any integers
and , any -degenerate graph can be equitably
partitioned into induced forests.
Each of these results implies the existence of a constant such that for
any , any planar graph has an equitable partition into induced
forests. This was conjectured by Wu, Zhang, and Li in 2013.Comment: 4 pages, final versio
Equitable partition of planar graphs
An equitable -partition of a graph is a collection of induced
subgraphs of such that
is a partition of and
for all . We prove that every planar graph admits an equitable
-partition into -degenerate graphs, an equitable -partition into
-degenerate graphs, and an equitable -partition into two forests and one
graph.Comment: 12 pages; revised; accepted to Discrete Mat
Equitable colorings of Kronecker products of graphs
AbstractFor a positive integer k, a graph G is equitably k-colorable if there is a mapping f:V(G)→{1,2,…,k} such that f(x)≠f(y) whenever xy∈E(G) and ||f−1(i)|−|f−1(j)||≤1 for 1≤i<j≤k. The equitable chromatic number of a graph G, denoted by χ=(G), is the minimum k such that G is equitably k-colorable. The equitable chromatic threshold of a graph G, denoted by χ=∗(G), is the minimum t such that G is equitably k-colorable for k≥t. The current paper studies equitable chromatic numbers of Kronecker products of graphs. In particular, we give exact values or upper bounds on χ=(G×H) and χ=∗(G×H) when G and H are complete graphs, bipartite graphs, paths or cycles
Reconfiguring Graph Colorings
Graph coloring has been studied for a long time and continues to receive
interest within the research community \cite{kubale2004graph}. It has applications
in scheduling \cite{daniel2004graph}, timetables, and compiler register
allocation \cite{lewis2015guide}. The most popular variant of graph coloring,
k-coloring, can be thought of as an assignment of colors to the vertices of a
graph such that adjacent vertices are assigned different colors.
Reconfiguration problems, typically defined on the solution space of search problems,
broadly ask whether one solution can be transformed to another solution using
step-by-step transformations, when constrained to one or more specific transformation
steps \cite{van2013complexity}. One well-studied reconfiguration problem is the
problem of deciding whether one k-coloring can be transformed to another k-coloring
by changing the color of one vertex at a time, while always maintaining a k-coloring
at each step.
We consider two variants of graph coloring: acyclic coloring and equitable
coloring, and their corresponding reconfiguration problems. A k-acylic coloring is
a k-coloring where there are more than two colors used by the vertices of each
cycle, and a k-equitable coloring is a k-coloring such that each color class, which is
defined as the set of all vertices with a particular color, is nearly the same
size as all others.
We show that reconfiguration of acyclic colorings is PSPACE-hard, and that for
non-bipartite graphs with chromatic number 3 there exist two k-acylic colorings
and such that there is no sequence of transformations that can
transform to . We also consider the problem of whether two
k-acylic colorings can be transformed to each other in at most steps, and
show that it is in XP, which is the class of algorithms that run in time
for some computable function and parameter , where in this
case the parameter is defined to be the length of the reconfiguration sequence
plus the length of the longest induced cycle.
We also show that the reconfiguration of equitable colorings is PSPACE-hard
and W[1]-hard with respect to the number of vertices with the same color. We
give polynomial-time algorithms for Reconfiguration of Equitable Colorings when
the number of colors used is two and also for paths when the number of colors
used is three