We investigate the expressive power of randomised first-order logic (BPFO) on restricted classes of structures. While BPFO is stronger than FO in general, even on structures with a built-in addition relation, we show that BPFO is not stronger than FO on structures with a unary vocabulary, nor on the class of equivalence relations. The same techniques can be applied to show that evenness of a linear order, and therefore graph connectivity, can not be defined in BPFO. Finally, we show that there is an FO[<]-definable query on word structures which can not be defined in BPFO[+1].
@InProceedings{eickmeyer:LIPIcs.CSL.2011.218, author = {Eickmeyer, Kord}, title = {{Non-Definability Results for Randomised First-Order Logic}}, booktitle = {Computer Science Logic (CSL'11) - 25th International Workshop/20th Annual Conference of the EACSL}, pages = {218--232}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-32-3}, ISSN = {1868-8969}, year = {2011}, volume = {12}, editor = {Bezem, Marc}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2011.218}, URN = {urn:nbn:de:0030-drops-32333}, doi = {10.4230/LIPIcs.CSL.2011.218}, annote = {Keywords: descriptive complexity, randomised logics, derandomisation} }
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