Diamond Subgraphs in the Reduction Graph of a One-Rule String Rewriting System

Abstract

In this paper, we study a certain case of a subgraph isomorphism problem. We consider the Hasse diagram of the lattice M_k (the unique lattice with k + 2 elements and one anti-chain of length k) and find the maximal k for which it is isomorphic to a subgraph of the reduction graph of a given one-rule string rewriting system. We obtain a complete characterization for this problem and show that there is a dichotomy. There are one-rule string rewriting systems for which the maximal such k is 2 and there are cases where there is no maximum. No other intermediate option is possible.