A homomorphism from a graph G to a graph H is a function from the vertices of G to the vertices of H that preserves edges. A homomorphism is surjective if it uses all of the vertices of H and it is a compaction if it uses all of the vertices of H and all of the non-loop edges of H . Hell and Nešetřil gave a complete characterisation of the complexity of deciding whether there is a homomorphism ...

This paper presents a graph-based knowledge representation and reasoning language. This language benefits from an important syntactic operation, which is called a graph homomorphism. This operation is sound and complete with respect to logical deduction. Hence, it is possible to do logical reasoning without using the language of logic but only graphical, thus visual, notions. This paper present...

Many problems in and areas of graph theory can be thought of in terms of graph homomorphisms. For graphs G and H, a homomorphism from G to H is a function φ : V (G)→ V (H) such that if xy ∈ E(G) then φ(x)φ(y) ∈ E(H). (See [1] for a comprehensive introduction.) Let Hom(G,H) be the set of homomorphisms from G to H. In this survey, we will be interested in counting homomorphisms from G to H and so...

The number of homomorphisms hom(G,Kk) from a graph G to the complete graph Kk is the value of the chromatic polynomial of G at a positive integer k. This motivates the following (cf. [3]): Definition 1 A sequence of graphs (Hk), k = (k1, . . . , kh) ∈ N , is strongly polynomial if for every graph G there is a polynomial p(G; x1, . . . , xh) such that hom(G,Hk) = p(G; k1, . . . , kh) for every k...

This paper presents a graph-based knowledge representation and reasoning language. This language benefits from an important syntactic operation, which is called a graph homomorphism. This operation is sound and complete with respect to logical deduction. Hence, it is possible to do logical reasoning without using the language of logic but only graphical, thus visual, notions. This paper present...

A graph G is said to be hom-idempotent if there is a homomorphism from G2 to G, and weakly hom-idempotent if for some n ≥ 1 there is a homomorphism from Gn+1 to Gn . We characterize both classes of graphs in terms of a special class of Cayley graphs called normal Cayley graphs. This allows us to construct, for any integer n, a Cayley graph G such that Gn+1 → Gn 6→ Gn−1, answering a question of ...

For 1 ≤ d ≤ k, let Kk/d be the graph with vertices 0, 1, · · · , k − 1, in which i ∼ j if d ≤ |i − j| ≤ k − d. The circular chromatic number χc(G) of a graph G is the minimum of those k/d for which G admits a homomorphism to Kk/d. The circular clique number ωc(G) of G is the maximum of those k/d for which Kk/d admits a homomorphism to G. A graph G is circular perfect if for every induced subgra...

A homomorphism from a graph X to a graph Y is an adjacency preserving mapping f : V (X)→ V (Y ). We consider a nonlocal game in which Alice and Bob are trying to convince a verifier with certainty that a graph X admits a homomorphism to Y . This is a generalization of the well-studied graph coloring game. Via systematic study of quantum homomorphisms we prove new results for graph coloring. Mos...

Given two graphs G = (VG, EG) and H = (VH , EH), a homomorphism from G to H is a function f : VG → VH such that for every uv ∈ EG, f(u)f(v) ∈ EH . A homomorphism from G to H is referred to as an H-colouring of G and the vertices of H are regarded as colours. The graph H is called the target of the homomorphism. These definitions extend to directed graphs by requiring that the mapping must prese...

A homomorphism from a graph G to a graph H is a vertex mapping f : VG → VH such that f(u) and f(v) form an edge in H whenever u and v form an edge in G. The H-Coloring problem is to test if a graph G allows a homomorphism to a given graph H. A well-known result of Hell and Nešetřil determines the computational complexity of this problem for any fixed graph H. We study a natural variant of this ...