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Documents authored by Sankaran, Abhisekh

Document
DOI: 10.4230/LIPIcs.CSL.2022.17
MSO Undecidability for Hereditary Classes of Unbounded Clique Width

Authors: Anuj Dawar and Abhisekh Sankaran

Published in: LIPIcs, Volume 216, 30th EACSL Annual Conference on Computer Science Logic (CSL 2022)

Abstract
Seese’s conjecture for finite graphs states that monadic second-order logic (MSO) is undecidable on all graph classes of unbounded clique-width. We show that to establish this it would suffice to show that grids of unbounded size can be interpreted in two families of graph classes: minimal hereditary classes of unbounded clique-width; and antichains of unbounded clique-width under the induced subgraph relation. We explore all the currently known classes of the former category and establish that grids of unbounded size can indeed be interpreted in them.
Cite as

Anuj Dawar and Abhisekh Sankaran. MSO Undecidability for Hereditary Classes of Unbounded Clique Width. In 30th EACSL Annual Conference on Computer Science Logic (CSL 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 216, pp. 17:1-17:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{dawar_et_al:LIPIcs.CSL.2022.17,
  author =	{Dawar, Anuj and Sankaran, Abhisekh},
  title =	{{MSO Undecidability for Hereditary Classes of Unbounded Clique Width}},
  booktitle =	{30th EACSL Annual Conference on Computer Science Logic (CSL 2022)},
  pages =	{17:1--17:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-218-1},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{216},
  editor =	{Manea, Florin and Simpson, Alex},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2022.17},
  URN =		{urn:nbn:de:0030-drops-157373},
  doi =		{10.4230/LIPIcs.CSL.2022.17},
  annote =	{Keywords: clique width, Seese’s conjecture, antichain, MSO interpretation, grid}
}
Document
DOI: 10.4230/LIPIcs.CSL.2021.18
Extension Preservation in the Finite and Prefix Classes of First Order Logic

Authors: Anuj Dawar and Abhisekh Sankaran

Published in: LIPIcs, Volume 183, 29th EACSL Annual Conference on Computer Science Logic (CSL 2021)

Abstract
It is well known that the classic Łoś-Tarski preservation theorem fails in the finite: there are first-order definable classes of finite structures closed under extensions which are not definable (in the finite) in the existential fragment of first-order logic. We strengthen this by constructing for every n, first-order definable classes of finite structures closed under extensions which are not definable with n quantifier alternations. The classes we construct are definable in the extension of Datalog with negation and indeed in the existential fragment of transitive-closure logic. This answers negatively an open question posed by Rosen and Weinstein.
Cite as

Anuj Dawar and Abhisekh Sankaran. Extension Preservation in the Finite and Prefix Classes of First Order Logic. In 29th EACSL Annual Conference on Computer Science Logic (CSL 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 183, pp. 18:1-18:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{dawar_et_al:LIPIcs.CSL.2021.18,
  author =	{Dawar, Anuj and Sankaran, Abhisekh},
  title =	{{Extension Preservation in the Finite and Prefix Classes of First Order Logic}},
  booktitle =	{29th EACSL Annual Conference on Computer Science Logic (CSL 2021)},
  pages =	{18:1--18:13},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-175-7},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{183},
  editor =	{Baier, Christel and Goubault-Larrecq, Jean},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2021.18},
  URN =		{urn:nbn:de:0030-drops-134520},
  doi =		{10.4230/LIPIcs.CSL.2021.18},
  annote =	{Keywords: finite model theory, preservation theorems, extension closed, composition, Datalog, Ehrenfeucht-Fraisse games}
}
Document
DOI: 10.4230/LIPIcs.CSL.2017.37
A Finitary Analogue of the Downward Löwenheim-Skolem Property

Authors: Abhisekh Sankaran

Published in: LIPIcs, Volume 82, 26th EACSL Annual Conference on Computer Science Logic (CSL 2017)

Abstract
We present a model-theoretic property of finite structures, that can be seen to be a finitary analogue of the well-studied downward Löwenheim-Skolem property from classical model theory. We call this property the *L-equivalent bounded substructure property*, denoted L-EBSP, where L is either FO or MSO. Intuitively, L-EBSP states that a large finite structure contains a small "logically similar" substructure, where logical similarity means indistinguishability with respect to sentences of L having a given quantifier nesting depth. It turns out that this simply stated property is enjoyed by a variety of classes of interest in computer science: examples include regular languages of words, trees (unordered, ordered or ranked) and nested words, and various classes of graphs, such as cographs, graph classes of bounded tree-depth, those of bounded shrub-depth and n-partite cographs. Further, L-EBSP remains preserved in the classes generated from the above by operations that are implementable using quantifier-free translation schemes. All of the aforementioned classes admit natural tree representations for their structures. We use this fact to show that the small and logically similar substructure of a large structure in any of these classes, can be computed in time linear in the size of the tree representation of the structure, giving linear time fixed parameter tractable (f.p.t.) algorithms for checking L-definable properties of the large structure. We conclude by presenting a strengthening of L-EBSP, that asserts "logical self-similarity at all scales" for a suitable notion of scale. We call this the *logical fractal* property and show that most of the classes mentioned above are indeed, logical fractals.
Cite as

Abhisekh Sankaran. A Finitary Analogue of the Downward Löwenheim-Skolem Property. In 26th EACSL Annual Conference on Computer Science Logic (CSL 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 82, pp. 37:1-37:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{sankaran:LIPIcs.CSL.2017.37,
  author =	{Sankaran, Abhisekh},
  title =	{{A Finitary Analogue of the Downward L\"{o}wenheim-Skolem Property}},
  booktitle =	{26th EACSL Annual Conference on Computer Science Logic (CSL 2017)},
  pages =	{37:1--37:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-045-3},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{82},
  editor =	{Goranko, Valentin and Dam, Mads},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2017.37},
  URN =		{urn:nbn:de:0030-drops-77074},
  doi =		{10.4230/LIPIcs.CSL.2017.37},
  annote =	{Keywords: downward Lowenheim-Skolem theorem, trees, nested words, tree-depth, cographs, tree representation, translation schemes, composition lemma, f.p.t., log}
}
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