We study an operator on classes of languages. For each class 𝒞, it produces a new class FO²(𝕀_𝒞) associated with a variant of two-variable first-order logic equipped with a signature 𝕀_𝒞 built from 𝒞. For 𝒞 = {∅, A*}, we obtain the usual FO²(<)} logic, equipped with linear order. For 𝒞 = {∅,{ε},A+,A*}, we get the variant FO²(<,+1), which also includes the successor predicate. If 𝒞 consists of all Boolean combinations of languages A*aA*, where a is a letter, we get the variant FO²(< ,Bet), which includes "between" relations. We prove a generic algebraic characterization of the classes FO^2(𝕀_𝒞). It elegantly generalizes those known for all the cases mentioned above. Moreover, it implies that if 𝒞 has decidable separation (plus some standard properties), then FO²2(𝕀_𝒞) has a decidable membership problem. We actually work with an equivalent definition of FO²(𝕀_𝒞) in terms of unary temporal logic. For each class 𝒞, we consider a variant TL(𝒞) of unary temporal logic whose future/past modalities depend on 𝒞 and such that TL(𝒞) = FO²(𝕀_𝒞). Finally, we also characterize FL(𝒞) and PL(𝒞), the pure-future and pure-past restrictions of TL(𝒞). Like for TL(𝒞), these characterizations imply that if 𝒞 is a class with decidable separation, then FL(𝒞) and PL(𝒞) have decidable membership.
@InProceedings{place_et_al:LIPIcs.CSL.2024.45, author = {Place, Thomas and Zeitoun, Marc}, title = {{A Generic Characterization of Generalized Unary Temporal Logic and Two-Variable First-Order Logic}}, booktitle = {32nd EACSL Annual Conference on Computer Science Logic (CSL 2024)}, pages = {45:1--45:23}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-310-2}, ISSN = {1868-8969}, year = {2024}, volume = {288}, editor = {Murano, Aniello and Silva, Alexandra}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2024.45}, URN = {urn:nbn:de:0030-drops-196888}, doi = {10.4230/LIPIcs.CSL.2024.45}, annote = {Keywords: Classes of regular languages, Generalized unary temporal logic, Generalized two-variable first-order logic, Generic decidable characterizations, Membership, Separation} }
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