It is well known that the classic Łoś-Tarski preservation theorem fails in the finite: there are first-order definable classes of finite structures closed under extensions which are not definable (in the finite) in the existential fragment of first-order logic. We strengthen this by constructing for every n, first-order definable classes of finite structures closed under extensions which are not definable with n quantifier alternations. The classes we construct are definable in the extension of Datalog with negation and indeed in the existential fragment of transitive-closure logic. This answers negatively an open question posed by Rosen and Weinstein.
@InProceedings{dawar_et_al:LIPIcs.CSL.2021.18, author = {Dawar, Anuj and Sankaran, Abhisekh}, title = {{Extension Preservation in the Finite and Prefix Classes of First Order Logic}}, booktitle = {29th EACSL Annual Conference on Computer Science Logic (CSL 2021)}, pages = {18:1--18:13}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-175-7}, ISSN = {1868-8969}, year = {2021}, volume = {183}, editor = {Baier, Christel and Goubault-Larrecq, Jean}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2021.18}, URN = {urn:nbn:de:0030-drops-134520}, doi = {10.4230/LIPIcs.CSL.2021.18}, annote = {Keywords: finite model theory, preservation theorems, extension closed, composition, Datalog, Ehrenfeucht-Fraisse games} }
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