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We consider the reachability problem for timed automata having diagonal constraints (like x - y < 5) as guards in transitions. The best algorithms for timed automata proceed by enumerating reachable sets of its configurations, stored in a data structure called "zones". Simulation relations between zones are essential to ensure termination and efficiency. The algorithm employs a simulation test Z <= Z' which ascertains that zone Z does not reach more states than zone Z', and hence further enumeration from Z is not necessary. No effective simulations are known for timed automata containing diagonal constraints as guards. We propose a simulation relation <=_{LU}^d for timed automata with diagonal constraints. On the negative side, we show that deciding Z not <=_{LU}^d Z' is NP-complete. On the positive side, we identify a witness for Z not <=_{LU}^d Z' and propose an algorithm to decide the existence of such a witness using an SMT solver. The shape of the witness reveals that the simulation test is likely to be efficient in practice.
@InProceedings{gastin_et_al:LIPIcs.CONCUR.2018.28,
author = {Gastin, Paul and Mukherjee, Sayan and Srivathsan, B.},
title = {{Reachability in Timed Automata with Diagonal Constraints}},
booktitle = {29th International Conference on Concurrency Theory (CONCUR 2018)},
pages = {28:1--28:17},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-087-3},
ISSN = {1868-8969},
year = {2018},
volume = {118},
editor = {Schewe, Sven and Zhang, Lijun},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2018.28},
URN = {urn:nbn:de:0030-drops-95660},
doi = {10.4230/LIPIcs.CONCUR.2018.28},
annote = {Keywords: Timed Automata, Reachability, Zones, Diagonal constraints}
}