We present syntactic characterisations for the union closed fragments of existential second-order logic and of logics with team semantics. Since union closure is a semantical and undecidable property, the normal form we introduce enables the handling and provides a better understanding of this fragment. We also introduce inclusion-exclusion games that turn out to be precisely the corresponding model-checking games. These games are not only interesting in their own right, but they also are a key factor towards building a bridge between the semantic and syntactic fragments. On the level of logics with team semantics we additionally present restrictions of inclusion-exclusion logic to capture the union closed fragment. Moreover, we define a team based atom that when adding it to first-order logic also precisely captures the union closed fragment of existential second-order logic which answers an open question by Galliani and Hella.
@InProceedings{hoelzel_et_al:LIPIcs.CSL.2020.25, author = {Hoelzel, Matthias and Wilke, Richard}, title = {{On the Union Closed Fragment of Existential Second-Order Logic and Logics with Team Semantics}}, booktitle = {28th EACSL Annual Conference on Computer Science Logic (CSL 2020)}, pages = {25:1--25:16}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-132-0}, ISSN = {1868-8969}, year = {2020}, volume = {152}, editor = {Fern\'{a}ndez, Maribel 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.CSL.2020.25}, URN = {urn:nbn:de:0030-drops-116681}, doi = {10.4230/LIPIcs.CSL.2020.25}, annote = {Keywords: Higher order logic, Existential second-order logic, Team semantics, Closure properties, Union closure, Model-checking games, Syntactic charactisations of semantical fragments} }
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