28,674 research outputs found
The set chromatic number of random graphs
In this paper we study the set chromatic number of a random graph
for a wide range of . We show that the set chromatic number, as a
function of , forms an intriguing zigzag shape
Chromatic number of the product of graphs, graph homomorphisms, Antichains and cofinal subsets of posets without AC
We have observations concerning the set theoretic strength of the following
combinatorial statements without the axiom of choice. 1. If in a partially
ordered set, all chains are finite and all antichains are countable, then the
set is countable. 2. If in a partially ordered set, all chains are finite and
all antichains have size , then the set has size
for any regular . 3. CS (Every partially
ordered set without a maximal element has two disjoint cofinal subsets). 4. CWF
(Every partially ordered set has a cofinal well-founded subset). 5. DT
(Dilworth's decomposition theorem for infinite p.o.sets of finite width). 6. If
the chromatic number of a graph is finite (say ), and the
chromatic number of another graph is infinite, then the chromatic
number of is . 7. For an infinite graph and a finite graph , if every finite subgraph of
has a homomorphism into , then so has . Further we study a few statements
restricted to linearly-ordered structures without the axiom of choice.Comment: Revised versio
Oriented coloring on recursively defined digraphs
Coloring is one of the most famous problems in graph theory. The coloring
problem on undirected graphs has been well studied, whereas there are very few
results for coloring problems on directed graphs. An oriented k-coloring of an
oriented graph G=(V,A) is a partition of the vertex set V into k independent
sets such that all the arcs linking two of these subsets have the same
direction. The oriented chromatic number of an oriented graph G is the smallest
k such that G allows an oriented k-coloring. Deciding whether an acyclic
digraph allows an oriented 4-coloring is NP-hard. It follows, that finding the
chromatic number of an oriented graph is an NP-hard problem. This motivates to
consider the problem on oriented co-graphs. After giving several
characterizations for this graph class, we show a linear time algorithm which
computes an optimal oriented coloring for an oriented co-graph. We further
prove how the oriented chromatic number can be computed for the disjoint union
and order composition from the oriented chromatic number of the involved
oriented co-graphs. It turns out that within oriented co-graphs the oriented
chromatic number is equal to the length of a longest oriented path plus one. We
also show that the graph isomorphism problem on oriented co-graphs can be
solved in linear time.Comment: 14 page
On the set chromatic number of the join and comb product of graphs
A vertex coloring c : V(G) → of a non-trivial connected graph G is called a set coloring if NC(u) ≠NC(v) for any pair of adjacent vertices u and v. Here, NC(x) denotes the set of colors assigned to vertices adjacent to x. The set chromatic number of G, denoted by χs (G), is defined as the fewest number of colors needed to construct a set coloring of G. In this paper, we study the set chromatic number in relation to two graph operations: join and comb prdocut. We determine the set chromatic number of wheels and the join of a bipartite graph and a cycle, the join of two cycles, the join of a complete graph and a bipartite graph, and the join of two bipartite graphs. Moreover, we determine the set chromatic number of the comb product of a complete graph with paths, cycles, and large star graphs
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