Plane-walking automata were introduced by Salo & Törma to recognise languages of two-dimensional infinite words (subshifts), the counterpart of 4-way finite automata for two-dimensional finite words. We extend the model to allow for nondeterminism and alternation of quantifiers. We prove that the recognised subshifts form a strict subclass of sofic subshifts, and that the classes corresponding to existential and universal nondeterminism are incomparable and both larger that the deterministic class. We define a hierarchy of subshifts recognised by plane-walking automata with alternating quantifiers, which we conjecture to be strict.
@InProceedings{hellouindemenibus_et_al:LIPIcs.STACS.2025.48, author = {Hellouin de Menibus, Benjamin and Perrotin, Pac\^{o}me}, title = {{Subshifts Defined by Nondeterministic and Alternating Plane-Walking Automata}}, booktitle = {42nd International Symposium on Theoretical Aspects of Computer Science (STACS 2025)}, pages = {48:1--48:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-365-2}, ISSN = {1868-8969}, year = {2025}, volume = {327}, editor = {Beyersdorff, Olaf and Pilipczuk, Micha{\l} and Pimentel, Elaine and Thắng, Nguy\~{ê}n Kim}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2025.48}, URN = {urn:nbn:de:0030-drops-228540}, doi = {10.4230/LIPIcs.STACS.2025.48}, annote = {Keywords: Formal languages, Finite automata, Subshifts, Symbolic dynamics, Tilings} }
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