3 Search Results for "Quickert, Sandra"

Document
DOI: 10.4230/LIPIcs.STACS.2025.69
A Dichotomy Theorem for Ordinal Ranks in MSO

Authors: Damian Niwiński, Paweł Parys, and Michał Skrzypczak

Published in: LIPIcs, Volume 327, 42nd International Symposium on Theoretical Aspects of Computer Science (STACS 2025)

Abstract
We focus on formulae ∃X.φ(Y, X) of monadic second-order logic over the full binary tree, such that the witness X is a well-founded set. The ordinal rank rank(X) < ω₁ of such a set X measures its depth and branching structure. We search for the least upper bound for these ranks, and discover the following dichotomy depending on the formula φ. Let η_φ be the minimal ordinal such that, whenever an instance Y satisfies the formula, there is a witness X with rank(X) ≤ η_φ. Then η_φ is either strictly smaller than ω² or it reaches the maximal possible value ω₁. Moreover, it is decidable which of the cases holds. The result has potential for applications in a variety of ordinal-related problems, in particular it entails a result about the closure ordinal of a fixed-point formula.
Cite as

Damian Niwiński, Paweł Parys, and Michał Skrzypczak. A Dichotomy Theorem for Ordinal Ranks in MSO. In 42nd International Symposium on Theoretical Aspects of Computer Science (STACS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 327, pp. 69:1-69:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{niwinski_et_al:LIPIcs.STACS.2025.69,
  author =	{Niwi\'{n}ski, Damian and Parys, Pawe{\l} and Skrzypczak, Micha{\l}},
  title =	{{A Dichotomy Theorem for Ordinal Ranks in MSO}},
  booktitle =	{42nd International Symposium on Theoretical Aspects of Computer Science (STACS 2025)},
  pages =	{69:1--69:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-365-2},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{327},
  editor =	{Beyersdorff, Olaf and Pilipczuk, Micha{\l} and Pimentel, Elaine and Thắng, Nguy\~{ê}n Kim},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2025.69},
  URN =		{urn:nbn:de:0030-drops-228942},
  doi =		{10.4230/LIPIcs.STACS.2025.69},
  annote =	{Keywords: dichotomy result, limit ordinal, countable ordinals, nondeterministic tree automata}
}
Document
DOI: 10.4230/LIPIcs.STACS.2025.55
Modal Separation of Fixpoint Formulae

Authors: Jean Christoph Jung and Jędrzej Kołodziejski

Published in: LIPIcs, Volume 327, 42nd International Symposium on Theoretical Aspects of Computer Science (STACS 2025)

Abstract
Modal separability for modal fixpoint formulae is the problem to decide for two given modal fixpoint formulae φ,φ' whether there is a modal formula ψ that separates them, in the sense that φ ⊧ ψ and ψ ⊧ ¬φ'. We study modal separability and its special case modal definability over various classes of models, such as arbitrary models, finite models, trees, and models of bounded outdegree. Our main results are that modal separability is PSpace-complete over words, that is, models of outdegree ≤ 1, ExpTime-complete over unrestricted and over binary models, and 2-ExpTime-complete over models of outdegree bounded by some d ≥ 3. Interestingly, this latter case behaves fundamentally different from the other cases also in that modal logic does not enjoy the Craig interpolation property over this class. Motivated by this we study also the induced interpolant existence problem as a special case of modal separability, and show that it is coNExpTime-complete and thus harder than validity in the logic. Besides deciding separability, we also investigate the problem of efficient construction of separators. Finally, we consider in a case study the extension of modal fixpoint formulae by graded modalities and investigate separability by modal formulae and graded modal formulae.
Cite as

Jean Christoph Jung and Jędrzej Kołodziejski. Modal Separation of Fixpoint Formulae. In 42nd International Symposium on Theoretical Aspects of Computer Science (STACS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 327, pp. 55:1-55:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{jung_et_al:LIPIcs.STACS.2025.55,
  author =	{Jung, Jean Christoph and Ko{\l}odziejski, J\k{e}drzej},
  title =	{{Modal Separation of Fixpoint Formulae}},
  booktitle =	{42nd International Symposium on Theoretical Aspects of Computer Science (STACS 2025)},
  pages =	{55:1--55:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-365-2},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{327},
  editor =	{Beyersdorff, Olaf and Pilipczuk, Micha{\l} and Pimentel, Elaine and Thắng, Nguy\~{ê}n Kim},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2025.55},
  URN =		{urn:nbn:de:0030-drops-228804},
  doi =		{10.4230/LIPIcs.STACS.2025.55},
  annote =	{Keywords: Modal Logic, Fixpoint Logic, Separability, Interpolation}
}
Document
DOI: 10.4230/LIPIcs.CSL.2015.457
Deciding the First Levels of the Modal mu Alternation Hierarchy by Formula Construction

Authors: Karoliina Lehtinen and Sandra Quickert

Published in: LIPIcs, Volume 41, 24th EACSL Annual Conference on Computer Science Logic (CSL 2015)

Abstract
We construct, for any sentence of the modal mu calculus Psi, derived sentences in the modal fragment and the fragment without least fixpoints of the modal mu calculus such that Psi is equivalent to a formula in these fragments if and only if it is equivalent to these formulas. The formula without greatest fixpoints that Psi is equivalent to if and only if it is equivalent to any formula without greatest fixpoint is obtained by duality. This yields a constructive proof of decidability of the first levels of the modal mu alternation hierarchy. The blow-up incurred by turning Psi into the modal formula is shown to be necessary: there are modal formulas that can be expressed sub-exponentially more efficiently with the use of fixpoints. For the fragments with only greatest or least fixpoints however, as long as formulas are in disjunctive form, the transformation into a formula syntactically in these fragments does not increase the size of the formula.
Cite as

Karoliina Lehtinen and Sandra Quickert. Deciding the First Levels of the Modal mu Alternation Hierarchy by Formula Construction. In 24th EACSL Annual Conference on Computer Science Logic (CSL 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 41, pp. 457-471, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

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@InProceedings{lehtinen_et_al:LIPIcs.CSL.2015.457,
  author =	{Lehtinen, Karoliina and Quickert, Sandra},
  title =	{{Deciding the First Levels of the Modal mu Alternation Hierarchy by Formula Construction}},
  booktitle =	{24th EACSL Annual Conference on Computer Science Logic (CSL 2015)},
  pages =	{457--471},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-939897-90-3},
  ISSN =	{1868-8969},
  year =	{2015},
  volume =	{41},
  editor =	{Kreutzer, Stephan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2015.457},
  URN =		{urn:nbn:de:0030-drops-54316},
  doi =		{10.4230/LIPIcs.CSL.2015.457},
  annote =	{Keywords: modal mu calculus, fixpoint logic, alternation hierarchy}
}
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