The finite satisfiability problem of monadic second order logic is decidable only on classes of structures of bounded tree-width by the classic result of Seese. We prove that the following problem is decidable: Input: (i) A monadic second order logic sentence alpha, and (ii) a sentence beta in the two-variable fragment of first order logic extended with counting quantifiers. The vocabularies of alpha and beta may intersect. Output: Is there a finite structure which satisfies alpha and beta such that the restriction of the structure to the vocabulary of alpha has bounded tree-width? (The tree-width of the desired structure is not bounded.) As a consequence, we prove the decidability of the satisfiability problem by a finite structure of bounded tree-width of a logic MS^{exists card} extending monadic second order logic with linear cardinality constraints of the form |X_{1}|+...+|X_{r}| < |Y_{1}|+...+|Y_{s}| on the variables X_i, Y_j of the outer-most quantifier block. We prove the decidability of a similar extension of WS1S.
@InProceedings{kotek_et_al:LIPIcs.CSL.2016.13, author = {Kotek, Tomer and Veith, Helmut and Zuleger, Florian}, title = {{Monadic Second Order Finite Satisfiability and Unbounded Tree-Width}}, booktitle = {25th EACSL Annual Conference on Computer Science Logic (CSL 2016)}, pages = {13:1--13:20}, 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.2016.13}, URN = {urn:nbn:de:0030-drops-65537}, doi = {10.4230/LIPIcs.CSL.2016.13}, annote = {Keywords: Monadic Second Order Logic MSO, Two variable Fragment with Counting C2, Finite decidability, Unbounded Tree-width, WS1S with Cardinality Constraints} }