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DOI: 10.4230/LIPIcs.FSCD.2026.32

New and Formalized Proofs for Right-Forward Closures and Core Matrix Interpretations

Authors: René Thiemann, Dieter Hofbauer, Ulysse Le Huitouze, and Johannes Waldmann

Published in: LIPIcs, Volume 378, 11th International Conference on Formal Structures for Computation and Deduction (FSCD 2026)

Abstract

We provide new proofs of two important theorems for proving termination of term rewrite systems (TRSs), including a full formalization in Isabelle/HOL. We first consider Dershowitz' theorem that termination starting from arbitrary terms is equivalent to termination starting from terms in the right-forward closures of right-hand sides, provided that the TRS is right-linear or orthogonal. Our new proof deviates from the original one in that no reorderings of steps in infinite derivations are required, making it more precise in its argumentation. It also subsumes a later result that one can weaken orthogonality to locally confluent overlay TRSs. The second theorem is about matrix interpretations. These were introduced by Hofbauer and Waldmann for proving termination of string rewrite systems (SRSs), internally using the concept of a core. Subsequently, Endrullis, Waldmann and Zantema developed matrix interpretations for TRSs without using the idea of a core. Whereas matrix interpretations for TRSs have already been formalized several times, so far this was not the case for core SRS matrix interpretations. We not only provide such a formalization, but also extend core SRS matrix interpretations to TRSs. These new core matrix interpretations for TRSs generalize previous approaches.

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René Thiemann, Dieter Hofbauer, Ulysse Le Huitouze, and Johannes Waldmann. New and Formalized Proofs for Right-Forward Closures and Core Matrix Interpretations. In 11th International Conference on Formal Structures for Computation and Deduction (FSCD 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 378, pp. 32:1-32:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026)

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@InProceedings{thiemann_et_al:LIPIcs.FSCD.2026.32,
  author =	{Thiemann, Ren\'{e} and Hofbauer, Dieter and Le Huitouze, Ulysse and Waldmann, Johannes},
  title =	{{New and Formalized Proofs for Right-Forward Closures and Core Matrix Interpretations}},
  booktitle =	{11th International Conference on Formal Structures for Computation and Deduction (FSCD 2026)},
  pages =	{32:1--32:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-433-8},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{378},
  editor =	{Pfenning, Frank},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSCD.2026.32},
  URN =		{urn:nbn:de:0030-drops-263825},
  doi =		{10.4230/LIPIcs.FSCD.2026.32},
  annote =	{Keywords: Isabelle/HOL, Matrix Interpretations, Narrowing, Right-Forward Closures, Term Rewriting}
}

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DOI: 10.4230/LIPIcs.FSCD.2019.21

Sparse Tiling Through Overlap Closures for Termination of String Rewriting

Authors: Alfons Geser, Dieter Hofbauer, and Johannes Waldmann

Published in: LIPIcs, Volume 131, 4th International Conference on Formal Structures for Computation and Deduction (FSCD 2019)

Abstract

A strictly locally testable language is characterized by its set of admissible factors, prefixes and suffixes, called tiles. We over-approximate reachability sets in string rewriting by languages defined by sparse sets of tiles, containing only those that are reachable in derivations. Using the partial algebra defined by a tiling for semantic labeling, we obtain a transformational method for proving local termination. These algebras can be represented efficiently as finite automata of a certain shape. Using a known result on forward closures, and a new characterisation of overlap closures, we can automatically prove termination and relative termination, respectively. We report on experiments showing the strength of the method.

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Alfons Geser, Dieter Hofbauer, and Johannes Waldmann. Sparse Tiling Through Overlap Closures for Termination of String Rewriting. In 4th International Conference on Formal Structures for Computation and Deduction (FSCD 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 131, pp. 21:1-21:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{geser_et_al:LIPIcs.FSCD.2019.21,
  author =	{Geser, Alfons and Hofbauer, Dieter and Waldmann, Johannes},
  title =	{{Sparse Tiling Through Overlap Closures for Termination of String Rewriting}},
  booktitle =	{4th International Conference on Formal Structures for Computation and Deduction (FSCD 2019)},
  pages =	{21:1--21:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-107-8},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{131},
  editor =	{Geuvers, Herman},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSCD.2019.21},
  URN =		{urn:nbn:de:0030-drops-105282},
  doi =		{10.4230/LIPIcs.FSCD.2019.21},
  annote =	{Keywords: relative termination, semantic labeling, locally testable language, overlap closure}
}

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Documents authored by Hofbauer, Dieter

New and Formalized Proofs for Right-Forward Closures and Core Matrix Interpretations

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Sparse Tiling Through Overlap Closures for Termination of String Rewriting

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