Parikh Automata over Infinite Words

TL;DR

This paper extends Parikh automata to infinite words, exploring their expressiveness, closure properties, and decision problems, revealing a complex hierarchy with decidability results for certain acceptance conditions.

Contribution

It introduces infinite-word variants of Parikh automata, analyzes their expressiveness and decision problems, and highlights decidability and undecidability results across different acceptance conditions.

Findings

Most automata classes have incomparable expressiveness.

Emptiness is decidable for reachability and B"uchi Parikh automata.

Model checking is decidable for deterministic safety and co-B"uchi Parikh automata.

Abstract

Parikh automata extend finite automata by counters that can be tested for membership in a semilinear set, but only at the end of a run, thereby preserving many of the desirable algorithmic properties of finite automata. Here, we study the extension of the classical framework onto infinite inputs: We introduce reachability, safety, B\"uchi, and co-B\"uchi Parikh automata on infinite words and study expressiveness, closure properties, and the complexity of verification problems. We show that almost all classes of automata have pairwise incomparable expressiveness, both in the deterministic and the nondeterministic case; a result that sharply contrasts with the well-known hierarchy in the $\omega$-regular setting. Furthermore, emptiness is shown decidable for Parikh automata with reachability or B\"uchi acceptance, but undecidable for safety and co-B\"uchi acceptance. Most importantly, we show decidability of model checking with specifications given by deterministic Parikh automata with safety or co-B\"uchi acceptance, but also undecidability for all other types of automata. Finally, solving games is undecidable for all types.