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} }
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