Explorable Parity Automata

TL;DR

This paper introduces explorable automata, a generalization of HD automata allowing multiple simultaneous runs to resolve nondeterminism, and analyzes their recognition complexity and expressivity on finite and infinite words.

Contribution

It defines explorable automata, establishes complexity results for recognizing HD and explorable automata, and characterizes their expressivity within the parity index hierarchy.

Findings

Recognizing HD parity automata of fixed index among explorable ones is in PTime.

Recognizing explorable automata is ExpTime-complete for finite words and low index parity automata.

ExpTime-completeness results for {}-explorability on infinite words with safety and coBbuchi conditions.

Abstract

We define the class of explorable automata on finite or infinite words. This is a generalization of History-Deterministic (HD) automata, where this time non-deterministic choices can be resolved by building finitely many simultaneous runs instead of just one. We show that recognizing HD parity automata of fixed index among explorable ones is in PTime, thereby giving a strong link between the two notions. We then show that recognizing explorable automata is ExpTime-complete, in the case of finite words or parity automata up to index [0, 2]. Additionally, we define the notion of {\omega}-explorable automata on infinite words, where countably many runs can be used to resolve the non-deterministic choices. We show ExpTime-completeness for {\omega}-explorability of automata on infinite words for the safety and coB\"uchi acceptance conditions. We finally characterize the expressivity of ({\omega}-)explorable automata with respect to the parity index hierarchy.