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New Integrality Gap Results for the Firefighters Problem on Trees

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Book cover Approximation and Online Algorithms (WAOA 2016)

Part of the book series: Lecture Notes in Computer Science ((LNTCS,volume 10138))

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Abstract

In the firefighter problem on trees, we are given a tree \(G=(V,E)\) together with a vertex \(s \in V\) where the fire starts spreading. At each time step, the firefighters can pick one vertex while the fire spreads from burning vertices to all their neighbors that have not been picked. The process stops when the fire can no longer spread. The objective is to find a strategy that maximizes the total number of vertices that do not burn. This is a simple mathematical model, introduced in 1995, that abstracts the spreading nature of, for instance, fire, viruses, and ideas. The firefighter problem is NP-hard and admits a \((1-1/e)\) approximation via LP rounding. Recently, a PTAS was announced in [1].(The \((1-1/e)\) approximation remained the best until very recently when Adjiashvili et al. [1] showed a PTAS. Their PTAS does not bound the LP gap.)

The goal of this paper is to develop better understanding on the power of LP relaxations for the firefighter problem. We first show a matching lower bound of \((1-1/e+\epsilon )\) on the integrality gap of the canonical LP. This result relies on a powerful combinatorial gadget that can be used to derive integrality gap results in other related settings. Next, we consider the canonical LP augmented with simple additional constraints (as suggested by Hartke). We provide several evidences that these constraints improve the integrality gap of the canonical LP: (i) Extreme points of the new LP are integral for some known tractable instances and (ii) A natural family of instances that are bad for the canonical LP admits an improved approximation algorithm via the new LP. We conclude by presenting a 5 / 6 integrality gap instance for the new LP.

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Notes

  1. 1.

    The notion of disconnecting the vertices here is slightly non-standard.

  2. 2.

    Cai et al. [5] claimed an LP-respecting integrality gap of \((1-1/e)\), but many natural rounding algorithms in the context of this problem are not LP respecting, e.g. in [6].

References

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Authors and Affiliations

  1. Aalto University, Espoo, Finland

    Parinya Chalermsook

  2. Max-Planck-Institut für Informatik, Saarbrücken, Germany

    Parinya Chalermsook & Daniel Vaz

  3. Graduate School of Computer Science, Saarland University, Saarbrücken, Germany

    Daniel Vaz

Authors
  1. Parinya Chalermsook
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  2. Daniel Vaz
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Correspondence to Daniel Vaz .

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Editors and Affiliations

  1. Institut für Informatik, Christian-Albrechts-Universität, Kiel, Germany

    Klaus Jansen

  2. Istituto Dalle Molle di Studi sull’ Intelligenza Artificiale, Manno (Lugano), Switzerland

    Monaldo Mastrolilli

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Chalermsook, P., Vaz, D. (2017). New Integrality Gap Results for the Firefighters Problem on Trees. In: Jansen, K., Mastrolilli, M. (eds) Approximation and Online Algorithms. WAOA 2016. Lecture Notes in Computer Science(), vol 10138. Springer, Cham. https://doi.org/10.1007/978-3-319-51741-4_6

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  • DOI: https://doi.org/10.1007/978-3-319-51741-4_6

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  • Online ISBN: 978-3-319-51741-4

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