We consider two-variable first-order logic on finite words with a fixed number of quantifier alternations. We show that all languages with a neutral letter definable using the order and finite-degree predicates are also definable with the order predicate only. From this result we derive the separation of the alternation hierarchy of two-variable logic on this signature. Replacing finite-degree by arbitrary numerical predicates in the statement would entail a long standing conjecture on the circuit complexity of the addition function. Thus, this result can be viewed as a uniform version of this circuit lower bound.
@InProceedings{paperman:LIPIcs.CSL.2015.616, author = {Paperman, Charles}, title = {{Finite-Degree Predicates and Two-Variable First-Order Logic}}, booktitle = {24th EACSL Annual Conference on Computer Science Logic (CSL 2015)}, pages = {616--630}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-90-3}, ISSN = {1868-8969}, year = {2015}, volume = {41}, editor = {Kreutzer, Stephan}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2015.616}, URN = {urn:nbn:de:0030-drops-54420}, doi = {10.4230/LIPIcs.CSL.2015.616}, annote = {Keywords: First order logic, automata theory, semigroup, modular predicates} }
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