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Documents authored by Razgon, Igor

Document
DOI: 10.4230/LIPIcs.STACS.2024.48
FPT Approximation of Generalised Hypertree Width for Bounded Intersection Hypergraphs

Authors: Matthias Lanzinger and Igor Razgon

Published in: LIPIcs, Volume 289, 41st International Symposium on Theoretical Aspects of Computer Science (STACS 2024)

Abstract
Generalised hypertree width (ghw) is a hypergraph parameter that is central to the tractability of many prominent problems with natural hypergraph structure. Computing ghw of a hypergraph is notoriously hard. The decision version of the problem, checking whether ghw(H) ≤ k, is paraNP-hard when parameterised by k. Furthermore, approximation of ghw is at least as hard as approximation of Set-Cover, which is known to not admit any FPT approximation algorithms. Research in the computation of ghw so far has focused on identifying structural restrictions to hypergraphs - such as bounds on the size of edge intersections - that permit XP algorithms for ghw. Yet, even under these restrictions that problem has so far evaded any kind of FPT algorithm. In this paper we make the first step towards FPT algorithms for ghw by showing that the parameter can be approximated in FPT time for graphs of bounded edge intersection size. In concrete terms we show that there exists an FPT algorithm, parameterised by k and d, that for input hypergraph H with maximal cardinality of edge intersections d and integer k either outputs a tree decomposition with ghw(H) ≤ 4k(k+d+1)(2k-1), or rejects, in which case it is guaranteed that ghw(H) > k. Thus, in the special case of hypergraphs of bounded edge intersection, we obtain an FPT O(k³)-approximation algorithm for ghw.
Cite as

Matthias Lanzinger and Igor Razgon. FPT Approximation of Generalised Hypertree Width for Bounded Intersection Hypergraphs. In 41st International Symposium on Theoretical Aspects of Computer Science (STACS 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 289, pp. 48:1-48:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{lanzinger_et_al:LIPIcs.STACS.2024.48,
  author =	{Lanzinger, Matthias and Razgon, Igor},
  title =	{{FPT Approximation of Generalised Hypertree Width for Bounded Intersection Hypergraphs}},
  booktitle =	{41st International Symposium on Theoretical Aspects of Computer Science (STACS 2024)},
  pages =	{48:1--48:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-311-9},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{289},
  editor =	{Beyersdorff, Olaf and Kant\'{e}, Mamadou Moustapha and Kupferman, Orna and Lokshtanov, Daniel},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2024.48},
  URN =		{urn:nbn:de:0030-drops-197588},
  doi =		{10.4230/LIPIcs.STACS.2024.48},
  annote =	{Keywords: generalized hypertree width, hypergraphs, parameterized algorithms, approximation algorithms}
}
Document
DOI: 10.4230/LIPIcs.IPEC.2021.25
Classification of OBDD Size for Monotone 2-CNFs

Authors: Igor Razgon

Published in: LIPIcs, Volume 214, 16th International Symposium on Parameterized and Exact Computation (IPEC 2021)

Abstract
We introduce a new graph parameter called linear upper maximum induced matching width lu-mim width, denoted for a graph G by lu(G). We prove that the smallest size of the obdd for φ, the monotone 2-cnf corresponding to G, is sandwiched between 2^{lu(G)} and n^{O(lu(G))}. The upper bound is based on a combinatorial statement that might be of an independent interest. We show that the bounds in terms of this parameter are best possible. The new parameter is closely related to two existing parameters: linear maximum induced matching width (lmim width) and linear special induced matching width (lsim width). We prove that lu-mim width lies strictly in between these two parameters, being dominated by lsim width and dominating lmim width. We conclude that neither of the two existing parameters can be used instead of lu-mim width to characterize the size of obdds for monotone 2-cnfs and this justifies introduction of the new parameter.
Cite as

Igor Razgon. Classification of OBDD Size for Monotone 2-CNFs. In 16th International Symposium on Parameterized and Exact Computation (IPEC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 214, pp. 25:1-25:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{razgon:LIPIcs.IPEC.2021.25,
  author =	{Razgon, Igor},
  title =	{{Classification of OBDD Size for Monotone 2-CNFs}},
  booktitle =	{16th International Symposium on Parameterized and Exact Computation (IPEC 2021)},
  pages =	{25:1--25:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-216-7},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{214},
  editor =	{Golovach, Petr A. and Zehavi, Meirav},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.IPEC.2021.25},
  URN =		{urn:nbn:de:0030-drops-154081},
  doi =		{10.4230/LIPIcs.IPEC.2021.25},
  annote =	{Keywords: Ordered Binary Decision Diagrams, Monotone 2-CNFs, Width parameters of graphs, upper and lower bounds}
}
Document
DOI: 10.4230/LIPIcs.MFCS.2020.41
Fractional Covers of Hypergraphs with Bounded Multi-Intersection

Authors: Georg Gottlob, Matthias Lanzinger, Reinhard Pichler, and Igor Razgon

Published in: LIPIcs, Volume 170, 45th International Symposium on Mathematical Foundations of Computer Science (MFCS 2020)

Abstract
Fractional (hyper-)graph theory is concerned with the specific problems that arise when fractional analogues of otherwise integer-valued (hyper-)graph invariants are considered. The focus of this paper is on fractional edge covers of hypergraphs. Our main technical result generalizes and unifies previous conditions under which the size of the support of fractional edge covers is bounded independently of the size of the hypergraph itself. This allows us to extend previous tractability results for checking if the fractional hypertree width of a given hypergraph is ≤ k for some constant k. We also show how our results translate to fractional vertex covers.
Cite as

Georg Gottlob, Matthias Lanzinger, Reinhard Pichler, and Igor Razgon. Fractional Covers of Hypergraphs with Bounded Multi-Intersection. In 45th International Symposium on Mathematical Foundations of Computer Science (MFCS 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 170, pp. 41:1-41:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{gottlob_et_al:LIPIcs.MFCS.2020.41,
  author =	{Gottlob, Georg and Lanzinger, Matthias and Pichler, Reinhard and Razgon, Igor},
  title =	{{Fractional Covers of Hypergraphs with Bounded Multi-Intersection}},
  booktitle =	{45th International Symposium on Mathematical Foundations of Computer Science (MFCS 2020)},
  pages =	{41:1--41:14},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-159-7},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{170},
  editor =	{Esparza, Javier and Kr\'{a}l', Daniel},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2020.41},
  URN =		{urn:nbn:de:0030-drops-127317},
  doi =		{10.4230/LIPIcs.MFCS.2020.41},
  annote =	{Keywords: Fractional graph theory, fractional edge cover, fractional hypertree width, bounded multi-intersection, fractional cover, fractional vertex cover}
}
Document
DOI: 10.4230/LIPIcs.STACS.2010.2485
Treewidth Reduction for Constrained Separation and Bipartization Problems

Authors: Dániel Marx, Barry O'Sullivan, and Igor Razgon

Published in: LIPIcs, Volume 5, 27th International Symposium on Theoretical Aspects of Computer Science (2010)

Abstract
We present a method for reducing the treewidth of a graph while preserving all the minimal $s-t$ separators. This technique turns out to be very useful for establishing the fixed-parameter tractability of constrained separation and bipartization problems. To demonstrate the power of this technique, we prove the fixed-parameter tractability of a number of well-known separation and bipartization problems with various additional restrictions (e.g., the vertices being removed from the graph form an independent set). These results answer a number of open questions in the area of parameterized complexity.
Cite as

Dániel Marx, Barry O'Sullivan, and Igor Razgon. Treewidth Reduction for Constrained Separation and Bipartization Problems. In 27th International Symposium on Theoretical Aspects of Computer Science. Leibniz International Proceedings in Informatics (LIPIcs), Volume 5, pp. 561-572, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2010)

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@InProceedings{marx_et_al:LIPIcs.STACS.2010.2485,
  author =	{Marx, D\'{a}niel and O'Sullivan, Barry and Razgon, Igor},
  title =	{{Treewidth Reduction for Constrained Separation and Bipartization Problems}},
  booktitle =	{27th International Symposium on Theoretical Aspects of Computer Science},
  pages =	{561--572},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-939897-16-3},
  ISSN =	{1868-8969},
  year =	{2010},
  volume =	{5},
  editor =	{Marion, Jean-Yves and Schwentick, Thomas},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2010.2485},
  URN =		{urn:nbn:de:0030-drops-24850},
  doi =		{10.4230/LIPIcs.STACS.2010.2485},
  annote =	{Keywords: Fixed-parameter algorithms, graph separation problems, treewidth}
}
Document
DOI: 10.4230/DagSemProc.07281.4
Directed Feedback Vertex Set is Fixed-Parameter Tractable

Authors: Igor Razgon and Barry O'Sullivan

Published in: Dagstuhl Seminar Proceedings, Volume 7281, Structure Theory and FPT Algorithmics for Graphs, Digraphs and Hypergraphs (2007)

Abstract
We resolve positively a long standing open question regarding the fixed-parameter tractability of the parameterized Directed Feedback Vertex Set problem. In particular, we propose an algorithm which solves this problem in $O(8^kk!*poly(n))$.
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Igor Razgon and Barry O'Sullivan. Directed Feedback Vertex Set is Fixed-Parameter Tractable. In Structure Theory and FPT Algorithmics for Graphs, Digraphs and Hypergraphs. Dagstuhl Seminar Proceedings, Volume 7281, pp. 1-14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2007)

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@InProceedings{razgon_et_al:DagSemProc.07281.4,
  author =	{Razgon, Igor and O'Sullivan, Barry},
  title =	{{Directed Feedback Vertex Set is Fixed-Parameter Tractable}},
  booktitle =	{Structure Theory and FPT Algorithmics for Graphs, Digraphs and Hypergraphs},
  pages =	{1--14},
  series =	{Dagstuhl Seminar Proceedings (DagSemProc)},
  ISSN =	{1862-4405},
  year =	{2007},
  volume =	{7281},
  editor =	{Erik Demaine and Gregory Z. Gutin and Daniel Marx and Ulrike Stege},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.07281.4},
  URN =		{urn:nbn:de:0030-drops-12363},
  doi =		{10.4230/DagSemProc.07281.4},
  annote =	{Keywords: Directed FVS, Multicut, Directed Acyclic Graph (DAG)}
}
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