The liveness problem for timed automata asks if a given automaton has a run passing infinitely often through an accepting state. We show that unless P=NP, the liveness problem is more difficult than the reachability problem; more precisely, we exhibit a family of automata for which solving the reachability problem with the standard algorithm is in P but solving the liveness problem is NP-hard. This leads us to revisit the algorithmics for the liveness problem. We propose a notion of a witness for the fact that a timed automaton violates a liveness property. We give an algorithm for computing such a witness and compare it with the existing solutions.
@InProceedings{herbreteau_et_al:LIPIcs.FSTTCS.2016.48, author = {Herbreteau, Fr\'{e}d\'{e}ric and Srivathsan, B. and Tran, Thanh-Tung and Walukiewicz, Igor}, title = {{Why Liveness for Timed Automata Is Hard, and What We Can Do About It}}, booktitle = {36th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2016)}, pages = {48:1--48:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-027-9}, ISSN = {1868-8969}, year = {2016}, volume = {65}, editor = {Lal, Akash and Akshay, S. and Saurabh, Saket and Sen, Sandeep}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2016.48}, URN = {urn:nbn:de:0030-drops-68831}, doi = {10.4230/LIPIcs.FSTTCS.2016.48}, annote = {Keywords: Timed automata, model-checking, liveness invariant, state subsumption} }
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