Languages Representable by Vertex-Labeled Graphs

In this paper we study the properties of undirected vertex-labeled graphs and the limitations on the languages that they represent. As a main result of this paper we define the necessary and sufficient conditions for the languages to be representable by a class of undirected vertex-labeled graphs and its subclasses. We assume that all obtained results and techniques are transferable to the case of undirected edge-labeled graphs and might give us similar results. The simplicity of necessary conditions emphasizes the naturalness of the result. The proof of their sufficiency is quite non-trivial and it is based on a new notion of quasi-equivalence, that is significantly different from Myhill-Nerode equivalence and might not be reduced to it.