A natural digraph analogue of the graph-theoretic concept of an ‘independent set’ is that of an ‘acyclic set’, namely a set of vertices not spanning a directed cycle. Hence a digraph analogue of a graph coloring is a decomposition of the vertex set into acyclic sets and we say a digraph is uniquely n-colorable when this decomposition is unique up to relabeling. It was shown probabilistically in...

In this paper, we study homomorphisms of 2-edge-colored graphs, that is graphs with edges colored with two colors. We consider various graph classes (outerplanar graphs, partial 2-trees, partial 3-trees, planar graphs) and the problem is to find, for each class, the smallest number of vertices of a 2-edge-colored graph H such that each graph of the considered class admits a homomorphism to H.

of ZfiΓ-modules and ZH-homomorphisms will be called an augmentation sequence if A is a finitely generated Ziϊ-module. Since ZH is a noetherian ring, A is then a finitely presented Zff-module; also, the module B in an augmentation sequence is finitely presented, since it is isomorphic to a submodule of A. For instance, if L £ S is a tame link of m components, G = π^S — L), and G is the commutato...

An automaton A is called a retractable automaton if, for every subau-tomaton B of A, there is at least one homomorphism of A onto B which leaves the elements of B fixed (such homomorphism is called a retract ho-momorphism of A onto B). We say that a retractable automaton A=(A,X,δ) is Boolean-type if there exists a family {λ B | B is a subautomaton of A} of retract homomorphisms λ B of A such th...

In order to study the complexity of counting problems, several interesting frameworks have been proposed, such as Constraint Satisfaction Problems (#CSP) and Graph Homomorphisms. Recently, we proposed and explored a novel alternative framework, called Holant Problems. It is a refinement with a more explicit role for constraint functions. Both graph homomorphism and #CSP can be viewed as special...

For digraphs G and H , a homomorphism of G to H is a mapping f : V (G)→V (H) such that uv ∈ A(G) implies f(u)f(v) ∈ A(H). If, moreover, each vertex u ∈ V (G) is associated with costs ci(u), i ∈ V (H), then the cost of a homomorphism f is ∑ u∈V (G) cf(u)(u). For each fixed digraph H , the minimum cost homomorphism problem for H , denoted MinHOM(H), can be formulated as follows: Given an input di...

For digraphs D and H , a mapping f : V (D)→V (H) is a homomorphism of D to H if uv ∈ A(D) implies f(u)f(v) ∈ A(H). For a fixed digraph H , the homomorphism problem is to decide whether an input digraph D admits a homomorphism to H or not, and is denoted as HOM(H). An optimization version of the homomorphism problem was motivated by a realworld problem in defence logistics and was introduced in ...

In this paper we design FPT-algorithms for two parameterized problems. The first is List Digraph Homomorphism: given two digraphs G and H and a list of allowed vertices of H for every vertex of G, the question is whether there exists a homomorphism from G to H respecting the list constraints. The second problem is a variant of Multiway Cut, namely Min-Max Multiway Cut: given a graph G, a non-ne...

Property-testers are fast randomized algorithms for distinguishing between graphs (and other combinatorial structures) satisfying a certain property, from those that are far from satisfying it. In many cases one can design property-testers whose running time is in fact independent of the size of the input. In this paper we survey some recent results on testing graph properties. A common thread ...

The biologically inspired framework of port-graphs has been successfully used to specify complex systems. It is the basis of the PORGY modelling tool. To facilitate the specification of proof normalisation procedures via graph rewriting, in this paper we add higher-order features to the original port-graph syntax, along with a generalised notion of graph morphism. We provide a matching algorith...