We show that graphs generated by collapsible pushdown systems of level $2$ are tree-automatic. Even when we allow $\varepsilon$-contractions and add a reachability predicate (with regular constraints) for pairs of configurations, the structures remain tree-automatic. Hence, their \FO theories are decidable, even when expanded by a reachability predicate. As a corollary, we obtain the tree-automaticity of the second level of the Caucal-hierarchy.
@InProceedings{kartzow:LIPIcs.STACS.2010.2480, author = {Kartzow, Alexander}, title = {{Collapsible Pushdown Graphs of Level 2 are Tree-Automatic}}, booktitle = {27th International Symposium on Theoretical Aspects of Computer Science}, pages = {501--512}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-16-3}, ISSN = {1868-8969}, year = {2010}, volume = {5}, editor = {Marion, Jean-Yves and Schwentick, Thomas}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2010.2480}, URN = {urn:nbn:de:0030-drops-24801}, doi = {10.4230/LIPIcs.STACS.2010.2480}, annote = {Keywords: Tree-automatic structures, collapsible pushdown graphs, collapsible pushdown systems, first-order decidability, reachability} }
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