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Monadic-second order logic (MSO-logic) is successfully applied in both language theory and algorithm design. In the former, properties definable by MSO-formulas are exactly the regular properties on many structures like, most prominently, strings. In the latter, solving a problem for structures of bounded tree width is routinely done by defining it in terms of an MSO-formula and applying general formula-evaluation procedures like Courcelle's. The present paper furthers the study of second-order logics with close connections to language theory and algorithm design beyond MSO-logic. We introduce a logic that allows to expand a given structure with an existentially quantified tree decomposition of bounded width and test an MSO-definable property for the resulting expanded structure. It is proposed as a candidate for capturing the notion of "context-free graph properties" since it corresponds to the context-free languages on strings, has the same closure properties, and an alternative definition similar to the one of Chomsky and Schützenberger for context-free languages. Besides studying its language-theoretic aspects, we consider its expressive power as well as the algorithmics of its satisfiability and evaluation problems.
@InProceedings{elberfeld:LIPIcs.CSL.2016.17,
author = {Elberfeld, Michael},
title = {{Context-Free Graph Properties via Definable Decompositions}},
booktitle = {25th EACSL Annual Conference on Computer Science Logic (CSL 2016)},
pages = {17:1--17:16},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-022-4},
ISSN = {1868-8969},
year = {2016},
volume = {62},
editor = {Talbot, Jean-Marc and Regnier, Laurent},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2016.17},
URN = {urn:nbn:de:0030-drops-65575},
doi = {10.4230/LIPIcs.CSL.2016.17},
annote = {Keywords: finite model theory, monadic second-order logic, tree decomposition, context-free languages, expressive power}
}