We give an algebraic characterization of the quantifier alternation hierarchy in first-order two-variable logic on finite words. As a result, we obtain a new proof that this hierarchy is strict. We also show that the first two levels of the hierarchy have decidable membership problems, and conjecture an algebraic decision procedure for the other levels.
@InProceedings{straubing:LIPIcs.CSL.2011.525, author = {Straubing, Howard}, title = {{Algebraic Characterization of the Alternation Hierarchy in FO^2\lbrack\langle\rbrack on Finite Words}}, booktitle = {Computer Science Logic (CSL'11) - 25th International Workshop/20th Annual Conference of the EACSL}, pages = {525--537}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-32-3}, ISSN = {1868-8969}, year = {2011}, volume = {12}, editor = {Bezem, Marc}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2011.525}, URN = {urn:nbn:de:0030-drops-32549}, doi = {10.4230/LIPIcs.CSL.2011.525}, annote = {Keywords: automata, finite model theory} }
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