Flat Parametric Counter Automata

In this paper we study the reachability problem for parametric flat counter automata, in relation with the satisfiability problem of three fragments of integer arithmetic. The equivalence between non-parametric flat counter automata and Presburger arithmetic has been established previously by Comon and Jurski. We simplify their proof by introducing finite state automata defined over alphabets of a special kind of graphs (zigzags). This framework allows one to express also the reachability problem for parametric automata with one control loop as the satisfiability of a 1-parametric linear Diophantine systems. The latter problem is shown to be decidable, using a number-theoretic argument. In general, the reachability problem for parametric flat counter automata with more than one loops is shown to be undecidable, by reduction from Hilbert's Tenth Problem. Finally, we study the relation between flat counter automata, integer arithmetic, and another important class of computational devices, namely the 2-way reversal bounded counter machines.