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Documents authored by Reinald, Amadeus

Document
DOI: 10.4230/LIPIcs.WG.2026.7
Making an Oriented Graph Acyclic Using Inversions of Bounded or Prescribed Size

Authors: Jørgen Bang-Jensen, Frédéric Havet, Florian Hörsch, Clément Rambaud, Amadeus Reinald, and Caroline Silva

Published in: LIPIcs, Volume 376, 52nd International Workshop on Graph-Theoretic Concepts in Computer Science (WG 2026)

Abstract
Given an oriented graph D, the inversion of a subset X of vertices consists in reversing the orientation of all arcs with both endpoints in X. When the subset X is of size p (resp. at most p), this operation is called an (= p)-inversion (resp. (⩽ p)-inversion). Then, an oriented graph is (= p)-invertible if it can be made acyclic by a sequence of p-inversions. We observe that, for n = |V(D)|, deciding whether D is (= n-1)-invertible is equivalent to deciding whether D is acyclically pushable, and thus NP-complete. In all other cases, whenever p ≠ n-1, we construct a polynomial-time algorithm deciding (= p)-invertibility. We then consider the (= p)-inversion number, inv^{= p}(D) (resp. (⩽ p)-inversion number, inv^{⩽ p}(D)), defined as the minimum number of (= p)-inversions (resp. (⩽ p)-inversions) rendering D acyclic. We show that every (= p)-invertible digraph D satisfies inv^{= p}(D) ⩽ |A(D)| for every integer p ⩾ 2. When p is even, we moreover bound inv^{= p} by a (linear) function of the feedback arc set number, and rule out the existence of any bounding function for odd p. Finally, we study the complexity of deciding whether the (= p)-inversion number, or the (⩽ p)-inversion number, of a given oriented graph is at most a given integer k. For any fixed positive integer p ⩾ 2, when k is part of the input, we show that both problems are NP-hard even in tournaments. In general oriented graphs, we prove W[1]-hardness for both problems when parameterized by p, even for k = 1. In contrast, we exhibit polynomial kernels in p + k for both problems in tournaments.
Cite as

Jørgen Bang-Jensen, Frédéric Havet, Florian Hörsch, Clément Rambaud, Amadeus Reinald, and Caroline Silva. Making an Oriented Graph Acyclic Using Inversions of Bounded or Prescribed Size. In 52nd International Workshop on Graph-Theoretic Concepts in Computer Science (WG 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 376, pp. 7:1-7:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026)

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@InProceedings{bangjensen_et_al:LIPIcs.WG.2026.7,
  author =	{Bang-Jensen, J{\o}rgen and Havet, Fr\'{e}d\'{e}ric and H\"{o}rsch, Florian and Rambaud, Cl\'{e}ment and Reinald, Amadeus and Silva, Caroline},
  title =	{{Making an Oriented Graph Acyclic Using Inversions of Bounded or Prescribed Size}},
  booktitle =	{52nd International Workshop on Graph-Theoretic Concepts in Computer Science (WG 2026)},
  pages =	{7:1--7:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-430-7},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{376},
  editor =	{Goedgebeur, Jan and Rz\k{a}\.{z}ewski, Pawe{\l}},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.WG.2026.7},
  URN =		{urn:nbn:de:0030-drops-261733},
  doi =		{10.4230/LIPIcs.WG.2026.7},
  annote =	{Keywords: digraph, inversion, orientation, NP-hardness, acyclic, reconfiguration}
}
Document
Track A: Algorithms, Complexity and Games
DOI: 10.4230/LIPIcs.ICALP.2026.27
Plane Strong Connectivity Augmentation

Authors: Stéphane Bessy, Daniel Gonçalves, Amadeus Reinald, and Dimitrios M. Thilikos

Published in: LIPIcs, Volume 374, 53rd International Colloquium on Automata, Languages, and Programming (ICALP 2026)

Abstract
We investigate the problem of strong connectivity augmentation within plane oriented graphs. We show that deciding whether a plane oriented graph D can be augmented with (any number of) arcs X such that D+X is strongly connected, but still plane and oriented, is NP-hard. The hardness also holds for the planar variant. This question becomes trivial within plane (or planar) digraphs, like most connectivity augmentation problems without a budget constraint. The budgeted variant, Plane Strong Connectivity Augmentation (PSCA) considers a plane oriented graph D along with some integer k, and asks for an X of size at most k ensuring that D+X is strongly connected, while remaining plane and oriented. Our main result is a fixed-parameter tractable algorithm for PSCA, running in time 2^O(k) n² log n. The cornerstone of our procedure is a structural result showing that, for any fixed k, each face admits a bounded number of partial solutions "dominating" all others. Then, our algorithm for PSCA combines face-wise branching with a randomized reduction to the polynomial Minimum Dijoin problem, yielding a Monte-Carlo FPT algorithm, which we derandomize. To the best of our knowledge, this is the first FPT algorithm for a (hard) connectivity augmentation problem constrained by planarity.
Cite as

Stéphane Bessy, Daniel Gonçalves, Amadeus Reinald, and Dimitrios M. Thilikos. Plane Strong Connectivity Augmentation. In 53rd International Colloquium on Automata, Languages, and Programming (ICALP 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 374, pp. 27:1-27:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026)

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@InProceedings{bessy_et_al:LIPIcs.ICALP.2026.27,
  author =	{Bessy, St\'{e}phane and Gon\c{c}alves, Daniel and Reinald, Amadeus and Thilikos, Dimitrios M.},
  title =	{{Plane Strong Connectivity Augmentation}},
  booktitle =	{53rd International Colloquium on Automata, Languages, and Programming (ICALP 2026)},
  pages =	{27:1--27:22},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-428-4},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{374},
  editor =	{Bhattacharya, Sayan and Nanongkai, Danupon and Benedikt, Michael and Puppis, Gabriele},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2026.27},
  URN =		{urn:nbn:de:0030-drops-264163},
  doi =		{10.4230/LIPIcs.ICALP.2026.27},
  annote =	{Keywords: Connectivity augmentation, Directed graphs, Parameterized complexity}
}
Document
DOI: 10.4230/LIPIcs.STACS.2025.6
Twin-Width One

Authors: Jungho Ahn, Hugo Jacob, Noleen Köhler, Christophe Paul, Amadeus Reinald, and Sebastian Wiederrecht

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

Abstract
We investigate the structure of graphs of twin-width at most 1, and obtain the following results: - Graphs of twin-width at most 1 are permutation graphs. In particular they have an intersection model and a linear structure. - There is always a 1-contraction sequence closely following a given permutation diagram. - Based on a recursive decomposition theorem, we obtain a simple algorithm running in linear time that produces a 1-contraction sequence of a graph, or guarantees that it has twin-width more than 1. - We characterise distance-hereditary graphs based on their twin-width and deduce a linear time algorithm to compute optimal sequences on this class of graphs.
Cite as

Jungho Ahn, Hugo Jacob, Noleen Köhler, Christophe Paul, Amadeus Reinald, and Sebastian Wiederrecht. Twin-Width One. In 42nd International Symposium on Theoretical Aspects of Computer Science (STACS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 327, pp. 6:1-6:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{ahn_et_al:LIPIcs.STACS.2025.6,
  author =	{Ahn, Jungho and Jacob, Hugo and K\"{o}hler, Noleen and Paul, Christophe and Reinald, Amadeus and Wiederrecht, Sebastian},
  title =	{{Twin-Width One}},
  booktitle =	{42nd International Symposium on Theoretical Aspects of Computer Science (STACS 2025)},
  pages =	{6:1--6:19},
  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.2025.6},
  URN =		{urn:nbn:de:0030-drops-228319},
  doi =		{10.4230/LIPIcs.STACS.2025.6},
  annote =	{Keywords: Twin-width, Hereditary graph classes, Intersection model}
}
Document
PACE Solver Description
DOI: 10.4230/LIPIcs.IPEC.2023.38
PACE Solver Description: Touiouidth

Authors: Gaétan Berthe, Yoann Coudert-Osmont, Alexander Dobler, Laure Morelle, Amadeus Reinald, and Mathis Rocton

Published in: LIPIcs, Volume 285, 18th International Symposium on Parameterized and Exact Computation (IPEC 2023)

Abstract
We describe Touiouidth, a twin-width solver for the exact-track of the 2023 PACE Challenge: Twin Width. Our solver is based on a simple branch and bound algorithm with search space reductions and is implemented in C++.
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Gaétan Berthe, Yoann Coudert-Osmont, Alexander Dobler, Laure Morelle, Amadeus Reinald, and Mathis Rocton. PACE Solver Description: Touiouidth. In 18th International Symposium on Parameterized and Exact Computation (IPEC 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 285, pp. 38:1-38:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{berthe_et_al:LIPIcs.IPEC.2023.38,
  author =	{Berthe, Ga\'{e}tan and Coudert-Osmont, Yoann and Dobler, Alexander and Morelle, Laure and Reinald, Amadeus and Rocton, Mathis},
  title =	{{PACE Solver Description: Touiouidth}},
  booktitle =	{18th International Symposium on Parameterized and Exact Computation (IPEC 2023)},
  pages =	{38:1--38:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-305-8},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{285},
  editor =	{Misra, Neeldhara and Wahlstr\"{o}m, Magnus},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.IPEC.2023.38},
  URN =		{urn:nbn:de:0030-drops-194576},
  doi =		{10.4230/LIPIcs.IPEC.2023.38},
  annote =	{Keywords: Twinwidth, Pace Challenge}
}
Document
PACE Solver Description
DOI: 10.4230/LIPIcs.IPEC.2022.31
PACE Solver Description: DreyFVS

Authors: Gabriel Bathie, Gaétan Berthe, Yoann Coudert-Osmont, David Desobry, Amadeus Reinald, and Mathis Rocton

Published in: LIPIcs, Volume 249, 17th International Symposium on Parameterized and Exact Computation (IPEC 2022)

Abstract
We describe DreyFVS, a heuristic for Directed Feedback Vertex Set submitted to the 2022 edition of Parameterized Algorithms and Computational Experiments Challenge. The Directed Feedback Vertex Set problem asks to remove a minimal number of vertices from a digraph such that the resulting digraph is acyclic. Our algorithm first performs a guess on a reduced instance by leveraging the Sinkhorn-Knopp algorithm, to then improve this solution by pipelining two local search methods.
Cite as

Gabriel Bathie, Gaétan Berthe, Yoann Coudert-Osmont, David Desobry, Amadeus Reinald, and Mathis Rocton. PACE Solver Description: DreyFVS. In 17th International Symposium on Parameterized and Exact Computation (IPEC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 249, pp. 31:1-31:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{bathie_et_al:LIPIcs.IPEC.2022.31,
  author =	{Bathie, Gabriel and Berthe, Ga\'{e}tan and Coudert-Osmont, Yoann and Desobry, David and Reinald, Amadeus and Rocton, Mathis},
  title =	{{PACE Solver Description: DreyFVS}},
  booktitle =	{17th International Symposium on Parameterized and Exact Computation (IPEC 2022)},
  pages =	{31:1--31:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-260-0},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{249},
  editor =	{Dell, Holger and Nederlof, Jesper},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.IPEC.2022.31},
  URN =		{urn:nbn:de:0030-drops-173870},
  doi =		{10.4230/LIPIcs.IPEC.2022.31},
  annote =	{Keywords: Directed Feedback Vertex Set, Heuristic, Sinkhorn algorithm, Local search}
}
Document
DOI: 10.4230/LIPIcs.IPEC.2021.10
Twin-Width and Polynomial Kernels

Authors: Édouard Bonnet, Eun Jung Kim, Amadeus Reinald, Stéphan Thomassé, and Rémi Watrigant

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

Abstract
We study the existence of polynomial kernels for parameterized problems without a polynomial kernel on general graphs, when restricted to graphs of bounded twin-width. It was previously observed in [Bonnet et al., ICALP'21] that the problem k-Independent Set allows no polynomial kernel on graph of bounded twin-width by a very simple argument, which extends to several other problems such as k-Independent Dominating Set, k-Path, k-Induced Path, k-Induced Matching. In this work, we examine the k-Dominating Set and variants of k-Vertex Cover for the existence of polynomial kernels. As a main result, we show that k-Dominating Set does not admit a polynomial kernel on graphs of twin-width at most 4 under a standard complexity-theoretic assumption. The reduction is intricate, especially due to the effort to bring the twin-width down to 4, and it can be tweaked to work for Connected k-Dominating Set and Total k-Dominating Set with a slightly worse bound on the twin-width. On the positive side, we obtain a simple quadratic vertex kernel for Connected k-Vertex Cover and Capacitated k-Vertex Cover on graphs of bounded twin-width. These kernels rely on that graphs of bounded twin-width have Vapnik-Chervonenkis (VC) density 1, that is, for any vertex set X, the number of distinct neighborhoods in X is at most c⋅|X|, where c is a constant depending only on the twin-width. Interestingly the kernel applies to any graph class of VC density 1, and does not require a witness sequence. We also present a more intricate O(k^{1.5}) vertex kernel for Connected k-Vertex Cover. Finally we show that deciding if a graph has twin-width at most 1 can be done in polynomial time, and observe that most graph optimization/decision problems can be solved in polynomial time on graphs of twin-width at most 1.
Cite as

Édouard Bonnet, Eun Jung Kim, Amadeus Reinald, Stéphan Thomassé, and Rémi Watrigant. Twin-Width and Polynomial Kernels. In 16th International Symposium on Parameterized and Exact Computation (IPEC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 214, pp. 10:1-10:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{bonnet_et_al:LIPIcs.IPEC.2021.10,
  author =	{Bonnet, \'{E}douard and Kim, Eun Jung and Reinald, Amadeus and Thomass\'{e}, St\'{e}phan and Watrigant, R\'{e}mi},
  title =	{{Twin-Width and Polynomial Kernels}},
  booktitle =	{16th International Symposium on Parameterized and Exact Computation (IPEC 2021)},
  pages =	{10:1--10:16},
  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.2021.10},
  URN =		{urn:nbn:de:0030-drops-153932},
  doi =		{10.4230/LIPIcs.IPEC.2021.10},
  annote =	{Keywords: Twin-width, kernelization, lower bounds, Dominating Set}
}
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