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A probabilistic finite automaton (PFA) A is said to be regular-approximable with respect to (x,y), if there is a regular language that contains all words accepted by A with probability at least x+y, but does not contain any word accepted with probability at most x. We show that the problem of determining if a PFA A is regular-approximable with respect to (x,y) is not recursively enumerable. We then show that many tractable sub-classes of PFAs identified in the literature - hierarchical PFAs, polynomially ambiguous PFAs, and eventually weakly ergodic PFAs - are regular-approximable with respect to all (x,y). Establishing the regular-approximability of a PFA has the nice consequence that its value can be effectively approximated, and the emptiness problem can be decided under the assumption of isolation.
@InProceedings{chadha_et_al:LIPIcs.CSL.2018.14,
author = {Chadha, Rohit and Sistla, A. Prasad and Viswanathan, Mahesh},
title = {{Approximating Probabilistic Automata by Regular Languages}},
booktitle = {27th EACSL Annual Conference on Computer Science Logic (CSL 2018)},
pages = {14:1--14:23},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-088-0},
ISSN = {1868-8969},
year = {2018},
volume = {119},
editor = {Ghica, Dan R. and Jung, Achim},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2018.14},
URN = {urn:nbn:de:0030-drops-96815},
doi = {10.4230/LIPIcs.CSL.2018.14},
annote = {Keywords: Probabilistic Finite Automata, Regular Languages, Ambiguity}
}