Homomorphisms on graph-walking automata

Graph-walking automata (GWA) are a model for graph traversal using finite-state control: these automata move between the nodes of an input graph, following its edges. This paper investigates the effect of node-replacement graph homomorphisms on recognizability by these automata. It is not difficult to see that the family of graph languages recognized by GWA is closed under inverse homomorphisms. The main result of this paper is that, for $n$-state automata operating on graphs with $k$ labels of edge end-points, the inverse homomorphic images require GWA with $kn+O(1)$ states in the worst case. The second result is that already for tree-walking automata, the family they recognize is not closed under injective homomorphisms. Here the proof is based on an easy homomorphic characterization of regular tree languages.