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Automatic structures are structures that admit a finite presentation via automata. Their most prominent feature is that their theories are decidable. In the literature, one finds automatic structures with non-elementary theory (e.g., the complete binary tree with equal-level predicate) and automatic structures whose theories are at most 3-fold exponential (e.g., Presburger arithmetic or infinite automatic graphs of bounded degree). This observation led Durand-Gasselin to the question whether there are automatic structures of arbitrary high elementary complexity. We give a positive answer to this question. Namely, we show that for every h >=0 the forest of (infinitely many copies of) all finite trees of height at most h+2 is automatic and it's theory is complete for STA(*, exp_h(n, poly(n)), poly(n)), an alternating complexity class between h-fold exponential time and space. This exact determination of the complexity of the theory of these forests might be of independent interest.
@InProceedings{abuzaid_et_al:LIPIcs.CSL.2018.3,
author = {Abu Zaid, Faried and Kuske, Dietrich and Lindner, Peter},
title = {{Climbing up the Elementary Complexity Classes with Theories of Automatic Structures}},
booktitle = {27th EACSL Annual Conference on Computer Science Logic (CSL 2018)},
pages = {3:1--3:16},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-088-0},
ISSN = {1868-8969},
year = {2018},
volume = {119},
editor = {Ghica, Dan R. and Jung, Achim},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2018.3},
URN = {urn:nbn:de:0030-drops-96701},
doi = {10.4230/LIPIcs.CSL.2018.3},
annote = {Keywords: Automatic Structures, Complexity Theory, Model Theory}
}