We establish an exact logical characterization of linear time complexity of cellular automata of dimension d, for any fixed d: a set of pictures of dimension d belongs to this complexity class iff it is definable in existential second-order logic restricted to monotonic Horn formulas with built-in successor function and d+1 first-order variables. This logical characterization is optimal modulo an open problem in parallel complexity. Furthermore, its proof provides a systematic method for transforming an inductive formula defining some problem into a cellular automaton that computes it in linear time.
@InProceedings{bacquey_et_al:LIPIcs.ICALP.2017.99, author = {Bacquey, Nicolas and Grandjean, Etienne and Olive, Fr\'{e}d\'{e}ric}, title = {{Definability by Horn Formulas and Linear Time on Cellular Automata}}, booktitle = {44th International Colloquium on Automata, Languages, and Programming (ICALP 2017)}, pages = {99:1--99:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-041-5}, ISSN = {1868-8969}, year = {2017}, volume = {80}, editor = {Chatzigiannakis, Ioannis and Indyk, Piotr and Kuhn, Fabian and Muscholl, Anca}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2017.99}, URN = {urn:nbn:de:0030-drops-74174}, doi = {10.4230/LIPIcs.ICALP.2017.99}, annote = {Keywords: picture languages, linear time, cellular automata of any dimension, local induction, descriptive complexity, second-order logic, horn formulas, logic} }
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