Detectability of labeled weighted automata over monoids

Abstract

In this paper, by developing appropriate methods, we for the first time obtain characterization of four fundamental notions of detectability for general labeled weighted automata over monoids (denoted by mathcal {A}^{mathfrak {M}} for short), where the four notions are strong (periodic) detectability (SD and SPD) and weak (periodic) detectability (WD and WPD). The contributions of the current paper are as follows. Firstly, we formulate the notions of concurrent composition, observer, and detector for mathcal {A}^{mathfrak {M}}. Secondly, we use the concurrent composition to give a necessary and sufficient condition for SD, use the detector to give a necessary and sufficient condition for SPD, and use the observer to give necessary and sufficient conditions for WD and WPD, all for general mathcal {A}^{mathfrak {M}} without any assumption. Thirdly, we prove that for a labeled weighted automaton over monoid (mathbb {Q}^{k},+) (denoted by mathcal {A}^{mathbb {Q}^{k}}), its concurrent composition, observer, and detector can be computed in NP, 2-EXPTIME, and 2-EXPTIME, respectively, by developing novel connections between mathcal {A}^{mathbb {Q}^{k}} and the NP-complete exact path length problem (proven by [Nykänen and Ukkonen, 2002]) and a subclass of Presburger arithmetic. As a result, we prove that for mathcal {A}^{mathbb {Q}^{k}}, SD can be verified in coNP, while SPD, WD, and WPD can be verified in 2-EXPTIME. Particularly, for mathcal {A}^{mathbb {Q}^{k}} in which from every state, a distinct state can be reached through some unobservable, instantaneous path, detector mathcal {A}^{mathbb {Q}^{k}}_{det} can be computed in NP, and SPD can be verified in coNP. Finally, we prove that the problems of verifying SD and SPD of deterministic, deadlock-free, and divergence-free mathcal {A}^{mathbb {N}} over monoid (mathbb {N},+) are both coNP-hard. The original methods developed in this paper will provide foundations for characterizing other fundamental properties (e.g., diagnosability and opacity) in labeled weighted automata over monoids. In addition, in order to differentiate labeled weighted automata over monoids from labeled timed automata, we also initially explore detectability in labeled timed automata, and prove that the SD verification problem is PSPACE-complete, while WD and WPD are undecidable.