Climbing up the Elementary Complexity Classes with Theories of Automatic Structures

Authors Faried Abu Zaid, Dietrich Kuske, Peter Lindner

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Faried Abu Zaid
  • TU Ilmenau, Germany
Dietrich Kuske
  • TU Ilmenau, Germany
Peter Lindner
  • RWTH Aachen University, Germany

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Faried Abu Zaid, Dietrich Kuske, and Peter Lindner. Climbing up the Elementary Complexity Classes with Theories of Automatic Structures. In 27th EACSL Annual Conference on Computer Science Logic (CSL 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 119, pp. 3:1-3:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)


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.

Subject Classification

ACM Subject Classification
  • Theory of computation → Complexity theory and logic
  • Automatic Structures
  • Complexity Theory
  • Model Theory


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