We study the equational theory of Kleene algebra (KA) w.r.t. languages (here, meaning the equational theory of regular expressions where each letter maps to any language) by extending the algebraic signature with the language complement. This extension significantly enhances the expressive power of KA. In this paper, we present a finite relational semantics completely characterizing the equational theory w.r.t. languages, which extends the relational characterizations known for KA and for KA with top. Based on this relational semantics, we show that the equational theory w.r.t. languages is Π⁰₁-complete for KA with complement (with or without Kleene-star) and is PSPACE-complete if the complement only applies to variables or constants.
@InProceedings{nakamura:LIPIcs.CSL.2025.37, author = {Nakamura, Yoshiki}, title = {{Finite Relational Semantics for Language Kleene Algebra with Complement}}, booktitle = {33rd EACSL Annual Conference on Computer Science Logic (CSL 2025)}, pages = {37:1--37:23}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-362-1}, ISSN = {1868-8969}, year = {2025}, volume = {326}, editor = {Endrullis, J\"{o}rg and Schmitz, Sylvain}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2025.37}, URN = {urn:nbn:de:0030-drops-227944}, doi = {10.4230/LIPIcs.CSL.2025.37}, annote = {Keywords: Kleene algebra, Language model, Relational model, Complexity} }
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