Let 𝓟₁,...,𝓟ₙ be properties of graphs. A (𝓟₁,...,𝓟ₙ)-partition of a graph G is a partition {V₁,...,Vₙ} of V(G) such that, for each i = 1,...,n, the subgraph of G induced by $V_i$ has property $𝓟_i$. If a graph G has a unique (𝓟₁,...,𝓟ₙ)-partition we say it is uniquely (𝓟₁,...,𝓟ₙ)-partitionable. We establish best lower bounds for the order of uniquely (𝓟₁,...,𝓟ₙ)-partitionable graphs, for various choices of 𝓟₁,...,𝓟ₙ.
We prove: (1) that $ch_P(G) - χ_P(G)$ can be arbitrarily large, where $ch_P(G)$ and $χ_P(G)$ are P-choice and P-chromatic numbers, respectively, (2) the (P,L)-colouring version of Brooks' and Gallai's theorems.
A hereditary property R of graphs is said to be reducible if there exist hereditary properties P₁,P₂ such that G ∈ R if and only if the set of vertices of G can be partitioned into V(G) = V₁∪V₂ so that ⟨V₁⟩ ∈ P₁ and ⟨V₂⟩ ∈ P₂. The problem of the factorization of reducible properties into irreducible factors is investigated.
For a simple graph H, →H denotes the class of all graphs that admit homomorphisms to H (such classes of graphs are called hom-properties). We investigate hom-properties from the point of view of the lattice of hereditary properties. In particular, we are interested in characterization of maximal graphs belonging to →H. We also provide a description of graphs maximal with respect to reducible hom-properties and determine the maximum number of edges of graphs belonging to →H.
Vizing [15] and Erdős et al. [8] independently introduce the idea of considering list-colouring and k-choosability. In the both papers the choosability version of Brooks' theorem [4] was proved but the choosability version of Gallai's theorem [9] was proved independently by Thomassen [14] and by Kostochka et al. [11]. In [3] some extensions of these two basic theorems to (𝓟,k)-choosability have been proved. In this paper we prove some extensions of the well-known bounds for the 𝓟-chromatic number to the (𝓟,k)-choice number and then an extension of Brooks' theorem.
Let L be the set of all hereditary and additive properties of graphs. For P₁, P₂ ∈ L, the reducible property R = P₁∘P₂ is defined as follows: G ∈ R if and only if there is a partition V(G) = V₁∪ V₂ of the vertex set of G such that $⟨V₁⟩_G ∈ P₁$ and $⟨V₂⟩_G ∈ P₂$. The aim of this paper is to investigate the structure of the reducible properties of graphs with emphasis on the uniqueness of the decomposition of a reducible property into irreducible ones.
A property of graphs is a non-empty set of graphs. A property P is called hereditary if every subgraph of any graph with property P also has property P. Let P₁, ...,Pₙ be properties of graphs. We say that a graph G has property P₁∘...∘Pₙ if the vertex set of G can be partitioned into n sets V₁, ...,Vₙ such that the subgraph of G induced by V_i has property $P_i$; i = 1,..., n. A hereditary property R is said to be reducible if there exist two hereditary properties P₁ and P₂ such that R = P₁∘P₂. If P is a hereditary property, then a graph G is called P- maximal if G has property P but G+e does not have property P for every e ∈ E([G̅]). We present some general results on maximal graphs and also investigate P-maximal graphs for various specific choices of P, including reducible hereditary properties.
The purpose of this paper is to present some basic properties of 𝓟-dominating, 𝓟-independent, and 𝓟-irredundant sets in graphs which generalize well-known properties of dominating, independent and irredundant sets, respectively.
Let 𝓟₁,...,𝓟ₙ be properties of graphs. A (𝓟₁,...,𝓟ₙ)-partition of a graph G is a partition of the vertex set V(G) into subsets V₁, ...,Vₙ such that the subgraph $G[V_i]$ induced by $V_i$ has property $𝓟_i$; i = 1,...,n. A graph G is said to be uniquely (𝓟₁, ...,𝓟ₙ)-partitionable if G has exactly one (𝓟₁,...,𝓟ₙ)-partition. A property 𝓟 is called hereditary if every subgraph of every graph with property 𝓟 also has property 𝓟. If every graph that is a disjoint union of two graphs that have property 𝓟 also has property 𝓟, then we say that 𝓟 is additive. A property 𝓟 is called degenerate if there exists a bipartite graph that does not have property 𝓟. In this paper, we prove that if 𝓟₁,..., 𝓟ₙ are degenerate, additive, hereditary properties of graphs, then there exists a uniquely (𝓟₁,...,𝓟ₙ)-partitionable graph.
Chartrand and Kronk in 1969 showed that there are planar graphs whose vertices cannot be partitioned into two parts inducing acyclic subgraphs. In this note we show that the same is true even in the case when one of the partition classes is required to be triangle-free only.
In this paper we survey results and open problems on the structure of additive and hereditary properties of graphs. The important role of vertex partition problems, in particular the existence of uniquely partitionable graphs and reducible properties of graphs in this structure is emphasized. Many related topics, including questions on the complexity of related problems, are investigated.
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