The ARRIVAL problem is to decide the fate of a train moving along the edges of a directed graph, according to a simple (deterministic) pseudorandom walk. The problem is in NP∩coNP but not known to be in 𝖯. The currently best algorithms have runtime 2^Θ(n) where n is the number of vertices. This is not much better than just performing the pseudorandom walk. We develop a subexponential algorithm with runtime 2^O(√nlog n). We also give a polynomial-time algorithm if the graph is almost acyclic. Both results are derived from a new general approach to solve ARRIVAL instances.
@InProceedings{gartner_et_al:LIPIcs.ICALP.2021.69, author = {G\"{a}rtner, Bernd and Haslebacher, Sebastian and Hoang, Hung P.}, title = {{A Subexponential Algorithm for ARRIVAL}}, booktitle = {48th International Colloquium on Automata, Languages, and Programming (ICALP 2021)}, pages = {69:1--69:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-195-5}, ISSN = {1868-8969}, year = {2021}, volume = {198}, editor = {Bansal, Nikhil and Merelli, Emanuela and Worrell, James}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2021.69}, URN = {urn:nbn:de:0030-drops-141387}, doi = {10.4230/LIPIcs.ICALP.2021.69}, annote = {Keywords: Pseudorandom walks, reachability, graph games, switching systems} }
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