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Documents authored by Mestre, Julián

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Mestre, Julián

Document
DOI: 10.4230/LIPIcs.ISAAC.2022.7
Approximating the Minimum Logarithmic Arrangement Problem

Authors: Julián Mestre and Sergey Pupyrev

Published in: LIPIcs, Volume 248, 33rd International Symposium on Algorithms and Computation (ISAAC 2022)

Abstract
We study a graph reordering problem motivated by compressing massive graphs such as social networks and inverted indexes. Given a graph, G = (V, E), the Minimum Logarithmic Arrangement problem is to find a permutation, π, of the vertices that minimizes ∑_{(u, v) ∈ E} (1 + ⌊ lg |π(u) - π(v)| ⌋). This objective has been shown to be a good measure of how many bits are needed to encode the graph if the adjacency list of each vertex is encoded using relative positions of two consecutive neighbors under the π order in the list rather than using absolute indices or node identifiers, which requires at least lg n bits per edge. We show the first non-trivial approximation factor for this problem by giving a polynomial time 𝒪(log k)-approximation algorithm for graphs with treewidth k.
Cite as

Julián Mestre and Sergey Pupyrev. Approximating the Minimum Logarithmic Arrangement Problem. In 33rd International Symposium on Algorithms and Computation (ISAAC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 248, pp. 7:1-7:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{mestre_et_al:LIPIcs.ISAAC.2022.7,
  author =	{Mestre, Juli\'{a}n and Pupyrev, Sergey},
  title =	{{Approximating the Minimum Logarithmic Arrangement Problem}},
  booktitle =	{33rd International Symposium on Algorithms and Computation (ISAAC 2022)},
  pages =	{7:1--7:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-258-7},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{248},
  editor =	{Bae, Sang Won and Park, Heejin},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2022.7},
  URN =		{urn:nbn:de:0030-drops-172924},
  doi =		{10.4230/LIPIcs.ISAAC.2022.7},
  annote =	{Keywords: approximation algorithms, graph compression}
}
Document
DOI: 10.4230/LIPIcs.ISAAC.2022.36
Nested Active-Time Scheduling

Authors: Nairen Cao, Jeremy T. Fineman, Shi Li, Julián Mestre, Katina Russell, and Seeun William Umboh

Published in: LIPIcs, Volume 248, 33rd International Symposium on Algorithms and Computation (ISAAC 2022)

Abstract
The active-time scheduling problem considers the problem of scheduling preemptible jobs with windows (release times and deadlines) on a parallel machine that can schedule up to g jobs during each timestep. The goal in the active-time problem is to minimize the number of active steps, i.e., timesteps in which at least one job is scheduled. In this way, the active time models parallel scheduling when there is a fixed cost for turning the machine on at each discrete step. This paper presents a 9/5-approximation algorithm for a special case of the active-time scheduling problem in which job windows are laminar (nested). This result improves on the previous best 2-approximation for the general case.
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Nairen Cao, Jeremy T. Fineman, Shi Li, Julián Mestre, Katina Russell, and Seeun William Umboh. Nested Active-Time Scheduling. In 33rd International Symposium on Algorithms and Computation (ISAAC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 248, pp. 36:1-36:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{cao_et_al:LIPIcs.ISAAC.2022.36,
  author =	{Cao, Nairen and Fineman, Jeremy T. and Li, Shi and Mestre, Juli\'{a}n and Russell, Katina and Umboh, Seeun William},
  title =	{{Nested Active-Time Scheduling}},
  booktitle =	{33rd International Symposium on Algorithms and Computation (ISAAC 2022)},
  pages =	{36:1--36:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-258-7},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{248},
  editor =	{Bae, Sang Won and Park, Heejin},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2022.36},
  URN =		{urn:nbn:de:0030-drops-173214},
  doi =		{10.4230/LIPIcs.ISAAC.2022.36},
  annote =	{Keywords: Scheduling algorithms, Active time, Approximation algorithm}
}
Document
DOI: 10.4230/LIPIcs.ISAAC.2021.42
On the Extended TSP Problem

Authors: Julián Mestre, Sergey Pupyrev, and Seeun William Umboh

Published in: LIPIcs, Volume 212, 32nd International Symposium on Algorithms and Computation (ISAAC 2021)

Abstract
We initiate the theoretical study of Ext-TSP, a problem that originates in the area of profile-guided binary optimization. Given a graph G = (V, E) with positive edge weights w: E → R^+, and a non-increasing discount function f(⋅) such that f(1) = 1 and f(i) = 0 for i > k, for some parameter k that is part of the problem definition. The problem is to sequence the vertices V so as to maximize ∑_{(u, v) ∈ E} f(|d_u - d_v|)⋅ w(u,v), where d_v ∈ {1, …, |V|} is the position of vertex v in the sequence. We show that Ext-TSP is APX-hard to approximate in general and we give a (k+1)-approximation algorithm for general graphs and a PTAS for some sparse graph classes such as planar or treewidth-bounded graphs. Interestingly, the problem remains challenging even on very simple graph classes; indeed, there is no exact n^o(k) time algorithm for trees unless the ETH fails. We complement this negative result with an exact n^O(k) time algorithm for trees.
Cite as

Julián Mestre, Sergey Pupyrev, and Seeun William Umboh. On the Extended TSP Problem. In 32nd International Symposium on Algorithms and Computation (ISAAC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 212, pp. 42:1-42:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{mestre_et_al:LIPIcs.ISAAC.2021.42,
  author =	{Mestre, Juli\'{a}n and Pupyrev, Sergey and Umboh, Seeun William},
  title =	{{On the Extended TSP Problem}},
  booktitle =	{32nd International Symposium on Algorithms and Computation (ISAAC 2021)},
  pages =	{42:1--42:14},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-214-3},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{212},
  editor =	{Ahn, Hee-Kap and Sadakane, Kunihiko},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2021.42},
  URN =		{urn:nbn:de:0030-drops-154751},
  doi =		{10.4230/LIPIcs.ISAAC.2021.42},
  annote =	{Keywords: profile-guided optimization, approximation algorithms, bandwidth, TSP}
}
Document
DOI: 10.4230/LIPIcs.ISAAC.2017.37
Barrier Coverage with Non-uniform Lengths to Minimize Aggregate Movements

Authors: Serge Gaspers, Joachim Gudmundsson, Julián Mestre, and Stefan Rümmele

Published in: LIPIcs, Volume 92, 28th International Symposium on Algorithms and Computation (ISAAC 2017)

Abstract
Given a line segment I=[0,L], the so-called barrier, and a set of n sensors with varying ranges positioned on the line containing I, the barrier coverage problem is to move the sensors so that they cover I, while minimising the total movement. In the case when all the sensors have the same radius the problem can be solved in O(n log n) time (Andrews and Wang, Algorithmica 2017). If the sensors have different radii the problem is known to be NP-hard to approximate within a constant factor (Czyzowicz et al., ADHOC-NOW 2009). We strengthen this result and prove that no polynomial time \rho^{1-\epsilon}-approximation algorithm exists unless P=NP, where \rho is the ratio between the largest radius and the smallest radius. Even when we restrict the number of sensors that are allowed to move by a parameter k, the problem turns out to be W[1]-hard. On the positive side we show that a ((2+\epsilon)\rho+2/\epsilon)-approximation can be computed in O(n^3/\epsilon^2) time and we prove fixed-parameter tractability when parameterized by the total movement assuming all numbers in the input are integers.
Cite as

Serge Gaspers, Joachim Gudmundsson, Julián Mestre, and Stefan Rümmele. Barrier Coverage with Non-uniform Lengths to Minimize Aggregate Movements. In 28th International Symposium on Algorithms and Computation (ISAAC 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 92, pp. 37:1-37:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{gaspers_et_al:LIPIcs.ISAAC.2017.37,
  author =	{Gaspers, Serge and Gudmundsson, Joachim and Mestre, Juli\'{a}n and R\"{u}mmele, Stefan},
  title =	{{Barrier Coverage with Non-uniform Lengths to Minimize Aggregate Movements}},
  booktitle =	{28th International Symposium on Algorithms and Computation (ISAAC 2017)},
  pages =	{37:1--37:13},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-054-5},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{92},
  editor =	{Okamoto, Yoshio and Tokuyama, Takeshi},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2017.37},
  URN =		{urn:nbn:de:0030-drops-82591},
  doi =		{10.4230/LIPIcs.ISAAC.2017.37},
  annote =	{Keywords: Barrier coverage, Sensor movement, Approximation, Parameterized complexity}
}
Document
DOI: 10.4230/LIPIcs.ISAAC.2017.55
Precedence-Constrained Min Sum Set Cover

Authors: Jessica McClintock, Julián Mestre, and Anthony Wirth

Published in: LIPIcs, Volume 92, 28th International Symposium on Algorithms and Computation (ISAAC 2017)

Abstract
We introduce a version of the Min Sum Set Cover (MSSC) problem in which there are "AND" precedence constraints on the m sets. In the Precedence-Constrained Min Sum Set Cover (PCMSSC) problem, when interpreted as directed edges, the constraints induce an acyclic directed graph. PCMSSC models the aim of scheduling software tests to prioritize the rate of fault detection subject to dependencies between tests. Our greedy scheme for PCMSSC is similar to the approaches of Feige, Lovasz, and, Tetali for MSSC, and Chekuri and Motwani for precedence-constrained scheduling to minimize weighted completion time. With a factor-4 increase in approximation ratio, we reduce PCMSSC to the problem of finding a maximum-density precedence-closed sub-family of sets, where density is the ratio of sub-family union size to cardinality. We provide a greedy factor-sqrt m algorithm for maximizing density; on forests of in-trees, we show this algorithm finds an optimal solution. Harnessing an alternative greedy argument of Chekuri and Kumar for Maximum Coverage with Group Budget Constraints, on forests of out-trees, we design an algorithm with approximation ratio equal to maximum tree height. Finally, with a reduction from the Planted Dense Subgraph detection problem, we show that its conjectured hardness implies there is no polynomial-time algorithm for PCMSSC with approximation factor in O(m^{1/12-epsilon}).
Cite as

Jessica McClintock, Julián Mestre, and Anthony Wirth. Precedence-Constrained Min Sum Set Cover. In 28th International Symposium on Algorithms and Computation (ISAAC 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 92, pp. 55:1-55:12, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{mcclintock_et_al:LIPIcs.ISAAC.2017.55,
  author =	{McClintock, Jessica and Mestre, Juli\'{a}n and Wirth, Anthony},
  title =	{{Precedence-Constrained Min Sum Set Cover}},
  booktitle =	{28th International Symposium on Algorithms and Computation (ISAAC 2017)},
  pages =	{55:1--55:12},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-054-5},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{92},
  editor =	{Okamoto, Yoshio and Tokuyama, Takeshi},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2017.55},
  URN =		{urn:nbn:de:0030-drops-82648},
  doi =		{10.4230/LIPIcs.ISAAC.2017.55},
  annote =	{Keywords: planted dense subgraph, min sum set cover, precedence constrained}
}
Document
DOI: 10.4230/LIPIcs.IPEC.2016.13
Turbocharging Treewidth Heuristics

Authors: Serge Gaspers, Joachim Gudmundsson, Mitchell Jones, Julián Mestre, and Stefan Rümmele

Published in: LIPIcs, Volume 63, 11th International Symposium on Parameterized and Exact Computation (IPEC 2016)

Abstract
A widely used class of algorithms for computing tree decompositions of graphs are heuristics that compute an elimination order, i.e., a permutation of the vertex set. In this paper, we propose to turbocharge these heuristics. For a target treewidth k, suppose the heuristic has already computed a partial elimination order of width at most k, but extending it by one more vertex exceeds the target width k. At this moment of regret, we solve a subproblem which is to recompute the last c positions of the partial elimination order such that it can be extended without exceeding width k. We show that this subproblem is fixed-parameter tractable when parameterized by k and c, but it is para-NP-hard and W[1]-hard when parameterized by only k or c, respectively. Our experimental evaluation of the FPT algorithm shows that we can trade a reasonable increase of the running time for quality of the solution.
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Serge Gaspers, Joachim Gudmundsson, Mitchell Jones, Julián Mestre, and Stefan Rümmele. Turbocharging Treewidth Heuristics. In 11th International Symposium on Parameterized and Exact Computation (IPEC 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 63, pp. 13:1-13:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{gaspers_et_al:LIPIcs.IPEC.2016.13,
  author =	{Gaspers, Serge and Gudmundsson, Joachim and Jones, Mitchell and Mestre, Juli\'{a}n and R\"{u}mmele, Stefan},
  title =	{{Turbocharging Treewidth Heuristics}},
  booktitle =	{11th International Symposium on Parameterized and Exact Computation (IPEC 2016)},
  pages =	{13:1--13:13},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-023-1},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{63},
  editor =	{Guo, Jiong and Hermelin, Danny},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.IPEC.2016.13},
  URN =		{urn:nbn:de:0030-drops-69322},
  doi =		{10.4230/LIPIcs.IPEC.2016.13},
  annote =	{Keywords: tree decomposition, heuristic, fixed-parameter tractability, local search}
}
Document
DOI: 10.4230/LIPIcs.STACS.2010.2476
Improved Approximation Guarantees for Weighted Matching in the Semi-Streaming Model

Authors: Leah Epstein, Asaf Levin, Julián Mestre, and Danny Segev

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

Abstract
We study the maximum weight matching problem in the semi-streaming model, and improve on the currently best one-pass algorithm due to Zelke (Proc.\ STACS~'08, pages 669--680) by devising a deterministic approach whose performance guarantee is $4.91 + \eps$. In addition, we study {\em preemptive} online algorithms, a sub-class of one-pass algorithms where we are only allowed to maintain a feasible matching in memory at any point in time. All known results prior to Zelke's belong to this sub-class. We provide a lower bound of $4.967$ on the competitive ratio of any such deterministic algorithm, and hence show that future improvements will have to store in memory a set of edges which is not necessarily a feasible matching. We conclude by presenting an empirical study, conducted in order to compare the practical performance of our approach to that of previously suggested algorithms.
Cite as

Leah Epstein, Asaf Levin, Julián Mestre, and Danny Segev. Improved Approximation Guarantees for Weighted Matching in the Semi-Streaming Model. In 27th International Symposium on Theoretical Aspects of Computer Science. Leibniz International Proceedings in Informatics (LIPIcs), Volume 5, pp. 347-358, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2010)

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@InProceedings{epstein_et_al:LIPIcs.STACS.2010.2476,
  author =	{Epstein, Leah and Levin, Asaf and Mestre, Juli\'{a}n and Segev, Danny},
  title =	{{Improved Approximation Guarantees for Weighted Matching in the Semi-Streaming Model}},
  booktitle =	{27th International Symposium on Theoretical Aspects of Computer Science},
  pages =	{347--358},
  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.2010.2476},
  URN =		{urn:nbn:de:0030-drops-24766},
  doi =		{10.4230/LIPIcs.STACS.2010.2476},
  annote =	{Keywords: Approximation guarantees, semi-streaming model, one-pass algorithm}
}
Document
DOI: 10.4230/LIPIcs.STACS.2009.1841
Improved Approximations for Guarding 1.5-Dimensional Terrains

Authors: Khaled Elbassioni, Erik Krohn, Domagoj Matijevic, Julian Mestre, and Domagoj Severdija

Published in: LIPIcs, Volume 3, 26th International Symposium on Theoretical Aspects of Computer Science (2009)

Abstract
We present a 4-approximation algorithm for the problem of placing the fewest guards on a 1.5D terrain so that every point of the terrain is seen by at least one guard. This improves on the currently best approximation factor of 5 (J. King, 2006). Unlike most of the previous techniques, our method is based on rounding the linear programming relaxation of the corresponding covering problem. Besides the simplicity of the analysis, which mainly relies on decomposing the constraint matrix of the LP into totally balanced matrices, our algorithm, unlike previous work, generalizes to the weighted and partial versions of the basic problem.
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Khaled Elbassioni, Erik Krohn, Domagoj Matijevic, Julian Mestre, and Domagoj Severdija. Improved Approximations for Guarding 1.5-Dimensional Terrains. In 26th International Symposium on Theoretical Aspects of Computer Science. Leibniz International Proceedings in Informatics (LIPIcs), Volume 3, pp. 361-372, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2009)

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@InProceedings{elbassioni_et_al:LIPIcs.STACS.2009.1841,
  author =	{Elbassioni, Khaled and Krohn, Erik and Matijevic, Domagoj and Mestre, Julian and Severdija, Domagoj},
  title =	{{Improved Approximations for Guarding 1.5-Dimensional Terrains}},
  booktitle =	{26th International Symposium on Theoretical Aspects of Computer Science},
  pages =	{361--372},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-939897-09-5},
  ISSN =	{1868-8969},
  year =	{2009},
  volume =	{3},
  editor =	{Albers, Susanne and Marion, Jean-Yves},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2009.1841},
  URN =		{urn:nbn:de:0030-drops-18410},
  doi =		{10.4230/LIPIcs.STACS.2009.1841},
  annote =	{Keywords: Covering problems, Guarding 1.5-terrains, Approximation algorithms, Linear programming, Totally balanced matrices}
}
Document
Extended Abstract
DOI: 10.4230/LIPIcs.STACS.2008.1315
Lagrangian Relaxation and Partial Cover (Extended Abstract)

Authors: Julián Mestre

Published in: LIPIcs, Volume 1, 25th International Symposium on Theoretical Aspects of Computer Science (2008)

Abstract
Lagrangian relaxation has been used extensively in the design of approximation algorithms. This paper studies its strengths and limitations when applied to Partial Cover. We show that for Partial Cover in general no algorithm that uses Lagrangian relaxation and a Lagrangian Multiplier Preserving (LMP) $alpha$-approximation as a black box can yield an approximation factor better than~$frac{4}{3} alpha$. This matches the upper bound given by K"onemann et al. (ESA 2006, pages 468--479). Faced with this limitation we study a specific, yet broad class of covering problems: Partial Totally Balanced Cover. By carefully analyzing the inner workings of the LMP algorithm we are able to give an almost tight characterization of the integrality gap of the standard linear relaxation of the problem. As a consequence we obtain improved approximations for the Partial version of Multicut and Path Hitting on Trees, Rectangle Stabbing, and Set Cover with $ ho$-Blocks.
Cite as

Julián Mestre. Lagrangian Relaxation and Partial Cover (Extended Abstract). In 25th International Symposium on Theoretical Aspects of Computer Science. Leibniz International Proceedings in Informatics (LIPIcs), Volume 1, pp. 539-550, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2008)

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@InProceedings{mestre:LIPIcs.STACS.2008.1315,
  author =	{Mestre, Juli\'{a}n},
  title =	{{Lagrangian Relaxation and Partial Cover}},
  booktitle =	{25th International Symposium on Theoretical Aspects of Computer Science},
  pages =	{539--550},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-939897-06-4},
  ISSN =	{1868-8969},
  year =	{2008},
  volume =	{1},
  editor =	{Albers, Susanne and Weil, Pascal},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2008.1315},
  URN =		{urn:nbn:de:0030-drops-13156},
  doi =		{10.4230/LIPIcs.STACS.2008.1315},
  annote =	{Keywords: Lagrangian Relaxation, Partial Cover, Primal-Dual Algorithms}
}

Mestre, Julian

Document
DOI: 10.4230/LIPIcs.ISAAC.2022.7
Approximating the Minimum Logarithmic Arrangement Problem

Authors: Julián Mestre and Sergey Pupyrev

Published in: LIPIcs, Volume 248, 33rd International Symposium on Algorithms and Computation (ISAAC 2022)

Abstract
We study a graph reordering problem motivated by compressing massive graphs such as social networks and inverted indexes. Given a graph, G = (V, E), the Minimum Logarithmic Arrangement problem is to find a permutation, π, of the vertices that minimizes ∑_{(u, v) ∈ E} (1 + ⌊ lg |π(u) - π(v)| ⌋). This objective has been shown to be a good measure of how many bits are needed to encode the graph if the adjacency list of each vertex is encoded using relative positions of two consecutive neighbors under the π order in the list rather than using absolute indices or node identifiers, which requires at least lg n bits per edge. We show the first non-trivial approximation factor for this problem by giving a polynomial time 𝒪(log k)-approximation algorithm for graphs with treewidth k.
Cite as

Julián Mestre and Sergey Pupyrev. Approximating the Minimum Logarithmic Arrangement Problem. In 33rd International Symposium on Algorithms and Computation (ISAAC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 248, pp. 7:1-7:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{mestre_et_al:LIPIcs.ISAAC.2022.7,
  author =	{Mestre, Juli\'{a}n and Pupyrev, Sergey},
  title =	{{Approximating the Minimum Logarithmic Arrangement Problem}},
  booktitle =	{33rd International Symposium on Algorithms and Computation (ISAAC 2022)},
  pages =	{7:1--7:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-258-7},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{248},
  editor =	{Bae, Sang Won and Park, Heejin},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2022.7},
  URN =		{urn:nbn:de:0030-drops-172924},
  doi =		{10.4230/LIPIcs.ISAAC.2022.7},
  annote =	{Keywords: approximation algorithms, graph compression}
}
Document
DOI: 10.4230/LIPIcs.ISAAC.2022.36
Nested Active-Time Scheduling

Authors: Nairen Cao, Jeremy T. Fineman, Shi Li, Julián Mestre, Katina Russell, and Seeun William Umboh

Published in: LIPIcs, Volume 248, 33rd International Symposium on Algorithms and Computation (ISAAC 2022)

Abstract
The active-time scheduling problem considers the problem of scheduling preemptible jobs with windows (release times and deadlines) on a parallel machine that can schedule up to g jobs during each timestep. The goal in the active-time problem is to minimize the number of active steps, i.e., timesteps in which at least one job is scheduled. In this way, the active time models parallel scheduling when there is a fixed cost for turning the machine on at each discrete step. This paper presents a 9/5-approximation algorithm for a special case of the active-time scheduling problem in which job windows are laminar (nested). This result improves on the previous best 2-approximation for the general case.
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Nairen Cao, Jeremy T. Fineman, Shi Li, Julián Mestre, Katina Russell, and Seeun William Umboh. Nested Active-Time Scheduling. In 33rd International Symposium on Algorithms and Computation (ISAAC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 248, pp. 36:1-36:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{cao_et_al:LIPIcs.ISAAC.2022.36,
  author =	{Cao, Nairen and Fineman, Jeremy T. and Li, Shi and Mestre, Juli\'{a}n and Russell, Katina and Umboh, Seeun William},
  title =	{{Nested Active-Time Scheduling}},
  booktitle =	{33rd International Symposium on Algorithms and Computation (ISAAC 2022)},
  pages =	{36:1--36:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-258-7},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{248},
  editor =	{Bae, Sang Won and Park, Heejin},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2022.36},
  URN =		{urn:nbn:de:0030-drops-173214},
  doi =		{10.4230/LIPIcs.ISAAC.2022.36},
  annote =	{Keywords: Scheduling algorithms, Active time, Approximation algorithm}
}
Document
DOI: 10.4230/LIPIcs.ISAAC.2021.42
On the Extended TSP Problem

Authors: Julián Mestre, Sergey Pupyrev, and Seeun William Umboh

Published in: LIPIcs, Volume 212, 32nd International Symposium on Algorithms and Computation (ISAAC 2021)

Abstract
We initiate the theoretical study of Ext-TSP, a problem that originates in the area of profile-guided binary optimization. Given a graph G = (V, E) with positive edge weights w: E → R^+, and a non-increasing discount function f(⋅) such that f(1) = 1 and f(i) = 0 for i > k, for some parameter k that is part of the problem definition. The problem is to sequence the vertices V so as to maximize ∑_{(u, v) ∈ E} f(|d_u - d_v|)⋅ w(u,v), where d_v ∈ {1, …, |V|} is the position of vertex v in the sequence. We show that Ext-TSP is APX-hard to approximate in general and we give a (k+1)-approximation algorithm for general graphs and a PTAS for some sparse graph classes such as planar or treewidth-bounded graphs. Interestingly, the problem remains challenging even on very simple graph classes; indeed, there is no exact n^o(k) time algorithm for trees unless the ETH fails. We complement this negative result with an exact n^O(k) time algorithm for trees.
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Julián Mestre, Sergey Pupyrev, and Seeun William Umboh. On the Extended TSP Problem. In 32nd International Symposium on Algorithms and Computation (ISAAC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 212, pp. 42:1-42:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{mestre_et_al:LIPIcs.ISAAC.2021.42,
  author =	{Mestre, Juli\'{a}n and Pupyrev, Sergey and Umboh, Seeun William},
  title =	{{On the Extended TSP Problem}},
  booktitle =	{32nd International Symposium on Algorithms and Computation (ISAAC 2021)},
  pages =	{42:1--42:14},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-214-3},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{212},
  editor =	{Ahn, Hee-Kap and Sadakane, Kunihiko},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2021.42},
  URN =		{urn:nbn:de:0030-drops-154751},
  doi =		{10.4230/LIPIcs.ISAAC.2021.42},
  annote =	{Keywords: profile-guided optimization, approximation algorithms, bandwidth, TSP}
}
Document
DOI: 10.4230/LIPIcs.ISAAC.2017.37
Barrier Coverage with Non-uniform Lengths to Minimize Aggregate Movements

Authors: Serge Gaspers, Joachim Gudmundsson, Julián Mestre, and Stefan Rümmele

Published in: LIPIcs, Volume 92, 28th International Symposium on Algorithms and Computation (ISAAC 2017)

Abstract
Given a line segment I=[0,L], the so-called barrier, and a set of n sensors with varying ranges positioned on the line containing I, the barrier coverage problem is to move the sensors so that they cover I, while minimising the total movement. In the case when all the sensors have the same radius the problem can be solved in O(n log n) time (Andrews and Wang, Algorithmica 2017). If the sensors have different radii the problem is known to be NP-hard to approximate within a constant factor (Czyzowicz et al., ADHOC-NOW 2009). We strengthen this result and prove that no polynomial time \rho^{1-\epsilon}-approximation algorithm exists unless P=NP, where \rho is the ratio between the largest radius and the smallest radius. Even when we restrict the number of sensors that are allowed to move by a parameter k, the problem turns out to be W[1]-hard. On the positive side we show that a ((2+\epsilon)\rho+2/\epsilon)-approximation can be computed in O(n^3/\epsilon^2) time and we prove fixed-parameter tractability when parameterized by the total movement assuming all numbers in the input are integers.
Cite as

Serge Gaspers, Joachim Gudmundsson, Julián Mestre, and Stefan Rümmele. Barrier Coverage with Non-uniform Lengths to Minimize Aggregate Movements. In 28th International Symposium on Algorithms and Computation (ISAAC 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 92, pp. 37:1-37:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{gaspers_et_al:LIPIcs.ISAAC.2017.37,
  author =	{Gaspers, Serge and Gudmundsson, Joachim and Mestre, Juli\'{a}n and R\"{u}mmele, Stefan},
  title =	{{Barrier Coverage with Non-uniform Lengths to Minimize Aggregate Movements}},
  booktitle =	{28th International Symposium on Algorithms and Computation (ISAAC 2017)},
  pages =	{37:1--37:13},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-054-5},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{92},
  editor =	{Okamoto, Yoshio and Tokuyama, Takeshi},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2017.37},
  URN =		{urn:nbn:de:0030-drops-82591},
  doi =		{10.4230/LIPIcs.ISAAC.2017.37},
  annote =	{Keywords: Barrier coverage, Sensor movement, Approximation, Parameterized complexity}
}
Document
DOI: 10.4230/LIPIcs.ISAAC.2017.55
Precedence-Constrained Min Sum Set Cover

Authors: Jessica McClintock, Julián Mestre, and Anthony Wirth

Published in: LIPIcs, Volume 92, 28th International Symposium on Algorithms and Computation (ISAAC 2017)

Abstract
We introduce a version of the Min Sum Set Cover (MSSC) problem in which there are "AND" precedence constraints on the m sets. In the Precedence-Constrained Min Sum Set Cover (PCMSSC) problem, when interpreted as directed edges, the constraints induce an acyclic directed graph. PCMSSC models the aim of scheduling software tests to prioritize the rate of fault detection subject to dependencies between tests. Our greedy scheme for PCMSSC is similar to the approaches of Feige, Lovasz, and, Tetali for MSSC, and Chekuri and Motwani for precedence-constrained scheduling to minimize weighted completion time. With a factor-4 increase in approximation ratio, we reduce PCMSSC to the problem of finding a maximum-density precedence-closed sub-family of sets, where density is the ratio of sub-family union size to cardinality. We provide a greedy factor-sqrt m algorithm for maximizing density; on forests of in-trees, we show this algorithm finds an optimal solution. Harnessing an alternative greedy argument of Chekuri and Kumar for Maximum Coverage with Group Budget Constraints, on forests of out-trees, we design an algorithm with approximation ratio equal to maximum tree height. Finally, with a reduction from the Planted Dense Subgraph detection problem, we show that its conjectured hardness implies there is no polynomial-time algorithm for PCMSSC with approximation factor in O(m^{1/12-epsilon}).
Cite as

Jessica McClintock, Julián Mestre, and Anthony Wirth. Precedence-Constrained Min Sum Set Cover. In 28th International Symposium on Algorithms and Computation (ISAAC 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 92, pp. 55:1-55:12, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{mcclintock_et_al:LIPIcs.ISAAC.2017.55,
  author =	{McClintock, Jessica and Mestre, Juli\'{a}n and Wirth, Anthony},
  title =	{{Precedence-Constrained Min Sum Set Cover}},
  booktitle =	{28th International Symposium on Algorithms and Computation (ISAAC 2017)},
  pages =	{55:1--55:12},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-054-5},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{92},
  editor =	{Okamoto, Yoshio and Tokuyama, Takeshi},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2017.55},
  URN =		{urn:nbn:de:0030-drops-82648},
  doi =		{10.4230/LIPIcs.ISAAC.2017.55},
  annote =	{Keywords: planted dense subgraph, min sum set cover, precedence constrained}
}
Document
DOI: 10.4230/LIPIcs.IPEC.2016.13
Turbocharging Treewidth Heuristics

Authors: Serge Gaspers, Joachim Gudmundsson, Mitchell Jones, Julián Mestre, and Stefan Rümmele

Published in: LIPIcs, Volume 63, 11th International Symposium on Parameterized and Exact Computation (IPEC 2016)

Abstract
A widely used class of algorithms for computing tree decompositions of graphs are heuristics that compute an elimination order, i.e., a permutation of the vertex set. In this paper, we propose to turbocharge these heuristics. For a target treewidth k, suppose the heuristic has already computed a partial elimination order of width at most k, but extending it by one more vertex exceeds the target width k. At this moment of regret, we solve a subproblem which is to recompute the last c positions of the partial elimination order such that it can be extended without exceeding width k. We show that this subproblem is fixed-parameter tractable when parameterized by k and c, but it is para-NP-hard and W[1]-hard when parameterized by only k or c, respectively. Our experimental evaluation of the FPT algorithm shows that we can trade a reasonable increase of the running time for quality of the solution.
Cite as

Serge Gaspers, Joachim Gudmundsson, Mitchell Jones, Julián Mestre, and Stefan Rümmele. Turbocharging Treewidth Heuristics. In 11th International Symposium on Parameterized and Exact Computation (IPEC 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 63, pp. 13:1-13:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{gaspers_et_al:LIPIcs.IPEC.2016.13,
  author =	{Gaspers, Serge and Gudmundsson, Joachim and Jones, Mitchell and Mestre, Juli\'{a}n and R\"{u}mmele, Stefan},
  title =	{{Turbocharging Treewidth Heuristics}},
  booktitle =	{11th International Symposium on Parameterized and Exact Computation (IPEC 2016)},
  pages =	{13:1--13:13},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-023-1},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{63},
  editor =	{Guo, Jiong and Hermelin, Danny},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.IPEC.2016.13},
  URN =		{urn:nbn:de:0030-drops-69322},
  doi =		{10.4230/LIPIcs.IPEC.2016.13},
  annote =	{Keywords: tree decomposition, heuristic, fixed-parameter tractability, local search}
}
Document
DOI: 10.4230/LIPIcs.STACS.2010.2476
Improved Approximation Guarantees for Weighted Matching in the Semi-Streaming Model

Authors: Leah Epstein, Asaf Levin, Julián Mestre, and Danny Segev

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

Abstract
We study the maximum weight matching problem in the semi-streaming model, and improve on the currently best one-pass algorithm due to Zelke (Proc.\ STACS~'08, pages 669--680) by devising a deterministic approach whose performance guarantee is $4.91 + \eps$. In addition, we study {\em preemptive} online algorithms, a sub-class of one-pass algorithms where we are only allowed to maintain a feasible matching in memory at any point in time. All known results prior to Zelke's belong to this sub-class. We provide a lower bound of $4.967$ on the competitive ratio of any such deterministic algorithm, and hence show that future improvements will have to store in memory a set of edges which is not necessarily a feasible matching. We conclude by presenting an empirical study, conducted in order to compare the practical performance of our approach to that of previously suggested algorithms.
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Leah Epstein, Asaf Levin, Julián Mestre, and Danny Segev. Improved Approximation Guarantees for Weighted Matching in the Semi-Streaming Model. In 27th International Symposium on Theoretical Aspects of Computer Science. Leibniz International Proceedings in Informatics (LIPIcs), Volume 5, pp. 347-358, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2010)

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@InProceedings{epstein_et_al:LIPIcs.STACS.2010.2476,
  author =	{Epstein, Leah and Levin, Asaf and Mestre, Juli\'{a}n and Segev, Danny},
  title =	{{Improved Approximation Guarantees for Weighted Matching in the Semi-Streaming Model}},
  booktitle =	{27th International Symposium on Theoretical Aspects of Computer Science},
  pages =	{347--358},
  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.2010.2476},
  URN =		{urn:nbn:de:0030-drops-24766},
  doi =		{10.4230/LIPIcs.STACS.2010.2476},
  annote =	{Keywords: Approximation guarantees, semi-streaming model, one-pass algorithm}
}
Document
DOI: 10.4230/LIPIcs.STACS.2009.1841
Improved Approximations for Guarding 1.5-Dimensional Terrains

Authors: Khaled Elbassioni, Erik Krohn, Domagoj Matijevic, Julian Mestre, and Domagoj Severdija

Published in: LIPIcs, Volume 3, 26th International Symposium on Theoretical Aspects of Computer Science (2009)

Abstract
We present a 4-approximation algorithm for the problem of placing the fewest guards on a 1.5D terrain so that every point of the terrain is seen by at least one guard. This improves on the currently best approximation factor of 5 (J. King, 2006). Unlike most of the previous techniques, our method is based on rounding the linear programming relaxation of the corresponding covering problem. Besides the simplicity of the analysis, which mainly relies on decomposing the constraint matrix of the LP into totally balanced matrices, our algorithm, unlike previous work, generalizes to the weighted and partial versions of the basic problem.
Cite as

Khaled Elbassioni, Erik Krohn, Domagoj Matijevic, Julian Mestre, and Domagoj Severdija. Improved Approximations for Guarding 1.5-Dimensional Terrains. In 26th International Symposium on Theoretical Aspects of Computer Science. Leibniz International Proceedings in Informatics (LIPIcs), Volume 3, pp. 361-372, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2009)

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@InProceedings{elbassioni_et_al:LIPIcs.STACS.2009.1841,
  author =	{Elbassioni, Khaled and Krohn, Erik and Matijevic, Domagoj and Mestre, Julian and Severdija, Domagoj},
  title =	{{Improved Approximations for Guarding 1.5-Dimensional Terrains}},
  booktitle =	{26th International Symposium on Theoretical Aspects of Computer Science},
  pages =	{361--372},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-939897-09-5},
  ISSN =	{1868-8969},
  year =	{2009},
  volume =	{3},
  editor =	{Albers, Susanne and Marion, Jean-Yves},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2009.1841},
  URN =		{urn:nbn:de:0030-drops-18410},
  doi =		{10.4230/LIPIcs.STACS.2009.1841},
  annote =	{Keywords: Covering problems, Guarding 1.5-terrains, Approximation algorithms, Linear programming, Totally balanced matrices}
}
Document
Extended Abstract
DOI: 10.4230/LIPIcs.STACS.2008.1315
Lagrangian Relaxation and Partial Cover (Extended Abstract)

Authors: Julián Mestre

Published in: LIPIcs, Volume 1, 25th International Symposium on Theoretical Aspects of Computer Science (2008)

Abstract
Lagrangian relaxation has been used extensively in the design of approximation algorithms. This paper studies its strengths and limitations when applied to Partial Cover. We show that for Partial Cover in general no algorithm that uses Lagrangian relaxation and a Lagrangian Multiplier Preserving (LMP) $alpha$-approximation as a black box can yield an approximation factor better than~$frac{4}{3} alpha$. This matches the upper bound given by K"onemann et al. (ESA 2006, pages 468--479). Faced with this limitation we study a specific, yet broad class of covering problems: Partial Totally Balanced Cover. By carefully analyzing the inner workings of the LMP algorithm we are able to give an almost tight characterization of the integrality gap of the standard linear relaxation of the problem. As a consequence we obtain improved approximations for the Partial version of Multicut and Path Hitting on Trees, Rectangle Stabbing, and Set Cover with $ ho$-Blocks.
Cite as

Julián Mestre. Lagrangian Relaxation and Partial Cover (Extended Abstract). In 25th International Symposium on Theoretical Aspects of Computer Science. Leibniz International Proceedings in Informatics (LIPIcs), Volume 1, pp. 539-550, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2008)

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@InProceedings{mestre:LIPIcs.STACS.2008.1315,
  author =	{Mestre, Juli\'{a}n},
  title =	{{Lagrangian Relaxation and Partial Cover}},
  booktitle =	{25th International Symposium on Theoretical Aspects of Computer Science},
  pages =	{539--550},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-939897-06-4},
  ISSN =	{1868-8969},
  year =	{2008},
  volume =	{1},
  editor =	{Albers, Susanne and Weil, Pascal},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2008.1315},
  URN =		{urn:nbn:de:0030-drops-13156},
  doi =		{10.4230/LIPIcs.STACS.2008.1315},
  annote =	{Keywords: Lagrangian Relaxation, Partial Cover, Primal-Dual Algorithms}
}
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