Editorial

This special issue of Networks is dedicated to the INOC 2009 Conference, held in Pisa, Italy, April 26–29, 2009. As with theprevious editions, this fourth edition of the conference has received great interest within the network optimization community,with a large number of participants from some 30 countries. The conference program reached a high scientific level includingfour plenary talks by invited speakers of international stature on relevant network optimization themes, and 106 talks across24 contributed sessions and five invited sessions. The Scientific and the Organizing Committees of INOC 2009 were chairedby Maria Grazia Scutellà (Università di Pisa, Italy).

INOC is the conference of the EURO working group “European Network Optimization Group.” This meeting alternateswith the INFORMS Telecommunications Network Optimization Conference and takes place every 2 years. The first INOCconference was organized by Walid Ben-Ameur and the optimization team of the INT (France) in Evry-Paris, 2003, the secondone was organized and hosted by Luis Gouveia at the University of Lisbon in 2005, while the third edition was organized andchaired by Bernard Fortz in Spa, Belgium, in 2007.

Among the 27 papers submitted, 14 papers were accepted for inclusion in this special issue. We note with sorrow that, duringthe preparation of the issue, one of its contributors, Prof. Bruno Simeone, unexpectedly and prematurely passed away. Prof.Bruno Simeone was respected and loved in the Operations Research community, at the international level, for his outstandingscientific activity and his profound humanity. Recently, Prof. Bruno Simeone had started to study how network flow techniquescan resolve certain practically relevant optimization problems arising in the area of electoral systems. The result of this workis the nice survey presented in [7]. We are honoured that this contribution of Prof. Simeone is part of this volume, which wededicate to his memory.

The contents of the issue are organized as follows.

Adasme et al. [1] propose binary quadratically constrained quadratic programs for minimizing power subject to bit rate andsubcarrier-allocation constraints, over wireless Orthogonal Frequency Division Multiple Access. Semidefinite programmingrelaxations are compared with the Linear Programs obtained by applying the Fortet linearization method. The numerical resultsshow the computational gain obtained by using the proposed quadratic models.

Imahori et al. [2] establish the computational complexity of a node capacitated in-tree packing problem, which is very relevantin the context of sensor networks.

Bortnikov et al. [3] study the Load-Distance Balancing Problem, where a set of clients has to be assigned to a set of servers.Two versions of the problem are investigated, involving the minimization of the maximum delay and the minimization of theaverage delay, respectively. Computational complexity issues and approximation results are presented for some special cases.

Ben-Ameur and Biha [4] propose a polynomial time algorithm for the Cut Separator problem. They also propose someequivalent linear programming formulations, and derive an explicit short polyhedral description of the dominant of the cutseparator polytope.

Fouilhoux et al. [5] study the survivable hierarchical telecommunication network design problem. They propose an integer linearprogramming formulation and related valid inequalities, by investigating the computational complexity of the correspondingseparation problems.

Klopfenstein and Nace [6] investigate the polyhedral impacts of the robust optimization framework, proposed by Bertsimasand Sim, for the 0-1 knapsack problem. In particular, classical cover cuts are adapted to the robust knapsack problem.

Pukelsheim et al. [7] show that network flow models and algorithms are a very flexible and effective tool for the analysis andthe design of contemporary electoral systems.

DOI 10.1002/net.20474Published online 1 December 2011 in Wiley Online Library (wileyonlinelibrary.com).© 2011 Wiley Periodicals, Inc.

NETWORKS—2012—DOI 10.1002/net

Ljubic et al. [8] propose exact approaches to the Single-Source Network Loading problem. Various MIP formulations aswell as disaggregated flow formulations are presented, which are compared from a theoretical and also from a computationalperspective.

Kosuch et al. [9] consider a mixed integer bilevel problem with a probabilistic knapsack constraint in the first level. Byassuming a discrete probability space, the problem is transformed into a quadratic optimization problem, for which upperbounding approaches are devised.

An improved interior-point algorithm for the multicommodity flow problem, which is based on quadratic regularizations, isstudied by Castro and Cuesta [10]. Extensive computational results show that the proposed algorithm is more efficient thanthe nonregularized version.

Orlowski and Pioro [11] present a survey devoted to optimization problems arising in the design of survivable communicationnetworks.

Uchoa et al. [12] propose algorithms to compute lower and upper bounds for the Multi-Level Capacitated Minimum SpanningTree problem. Specifically, Branch-and-Cut and hybrid Local Search algorithms are presented, which are tested on benchmarkinstances from the literature.

Addis et al. [13] address a Two-level Location problem with modular node capacities, where location and dimensioning issuesare addressed simultaneously. A compact formulation is presented. Then, using discretization and Dantzig-Wolfe reformulationtechniques, an exact optimization approach is proposed and computationally tested.

Finally, recoverable robust shortest path problems are studied by Büsing [14] to deal with cost uncertainties. Computationalcomplexity and approximation issues are discussed.

Luis Gouveia and Maria Grazia ScutellàGuest Editors

Acknowledgments

We thank the authors for having submitted such high quality papers, and the referees for their excellent reviews. On behalf ofthe Organizing Committee of INOC 2009, we also thank all the people who helped to organize this conference. The supportprovided by EURO, ENOG, the University of Pisa, France Telecom R&D, Axiom and Nous Informatica has contributedsignificantly to the success of the event.

REFERENCES

[1] P. Adasme, A. Lisser, and I. Soto, A quadratic semidefinite relaxation approach for OFDMA resource allocation, Networks 59 (2012),3–12.

[2] S. Imahori, Y. Miyamoto, H. Hashimoto, Y. Kobayashi, M. Sasaki, and M. Yagiura, The complexity of the node capacitated in-treepacking problem, Networks 59 (2012), 13–21.

[3] E. Bortnikov, S. Khuller, J. Li, Y. Mansour, and J. Naor, The load-distance balancing problem, Networks 59 (2012), 22–29.[4] W. Ben-Ameur and M. Didi Biha, On the minimum cut separator problem, Networks 59 (2012), 30–36.[5] P. Fouilhoux, O.E. Karasan, A. Ridha Mahjoub, O. Özkök, and H. Yaman, Survivability in hierarchical telecommunications networks,

Networks 59 (2012), 37–58.[6] O. Klopfenstein and D. Nace, Cover inequalities for robust knapsack sets - Application to the robust bandwidth packing problem,

Networks 59 (2012), 59–72.[7] F. Pukelsheim, F. Ricca, A. Scozzari, P. Serani, and B. Simeone, Network flow methods for electoral systems, Networks 59 (2012),

73–88.[8] I. Ljubic, P. Putz, and J.J. Salazar-González, Exact approaches to the single-source network loading problem, Networks 59 (2012),

89–106.[9] S. Kosuch, P. Le Bodic, J. Leung, and A. Lisser, On a stochastic bilevel programming problem, Networks 59 (2012), 107–116.

[10] J. Castro and J. Cuesta, Improving an interior-point algorithm for multicommodity flows by quadratic regularizations, Networks 59(2012), 117–131.

[11] S. Orlowski and M. Pioro, On the complexity of column generation in network design with path-based survivability mechanisms,Networks 59 (2012), 132–147.

[12] E. Uchoa, T.A.M. Toffolo, M.C. de Souza, A.X. Martins, and R. Fukasawa, Branch-and-cut and hybrid local search for the multi-levelcapacitated minimum spanning tree problem, Networks 59 (2012), 148–160.

[13] B. Addis, G. Carello, and A. Ceselli, Exactly solving a two-level location problem with modular node capacities, Networks 59 (2012),161–180.

[14] C. Büsing, Recoverable robust shortest path problems, Networks 59 (2012), 181–189.

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