## Abstract

We address the fundamental network design problem of constructing approximate minimum spanners. Our contributions are for the distributed setting, providing both algorithmic and hardness results. Our main hardness result is that an -approximation for the minimum directed k-spanner problem for k ≥ 5 requires Ω(n/ log n) rounds using deterministic algorithms or Ω(n/ log n) rounds using randomized ones, in the Congest model of distributed computing. Combined with the constant-round O(n^{ϵ} )-approximation algorithm in the Local model of [Barenboim, Elkin and Gavoille, 2016], as well as a polylog-round (1 +)-approximation algorithm in the Local model that we show here, our lower bounds for the Congest model imply a strict separation between the Local and Congest models. Notably, to the best of our knowledge, this is the first separation between these models for a local approximation problem. Similarly, a separation between the directed and undirected cases is implied. We also prove a nearly-linear lower bound for the minimum weighted k-spanner problem for k ≥ 4, and we show lower bounds for the weighted 2-spanner problem. On the algorithmic side, apart from the aforementioned (1 + )- approximation algorithm for minimum k-spanners, our main contribution is a new distributed construction of minimum 2-spanners that uses only polynomial local computations. Our algorithm has a guaranteed approximation ratio of O(log(m/n)) for a graph with n vertices and m edges, which matches the best known ratio for polynomial time sequential algorithms [Kortsarz and Peleg, 1994], and is tight if we restrict ourselves to polynomial local computations. Our approach allows us to extend our algorithm to work also for the directed, weighted, and client-server variants of the problem. It also provides a Congest algorithm for the minimum dominating set problem, with a guaranteed O(log ∆) approximation ratio.

Original language | English |
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Title of host publication | PODC 2018 - Proceedings of the 2018 ACM Symposium on Principles of Distributed Computing |

Publisher | Association for Computing Machinery |

Pages | 139-148 |

Number of pages | 10 |

ISBN (Print) | 9781450357951 |

DOIs | |

State | Published - 23 Jul 2018 |

Externally published | Yes |

Event | 37th ACM SIGACT-SIGOPS Symposium on Principles of Distributed Computing, PODC 2018 - Egham, United Kingdom Duration: 23 Jul 2018 → 27 Jul 2018 |

### Publication series

Name | Proceedings of the Annual ACM Symposium on Principles of Distributed Computing |
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### Conference

Conference | 37th ACM SIGACT-SIGOPS Symposium on Principles of Distributed Computing, PODC 2018 |
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Country/Territory | United Kingdom |

City | Egham |

Period | 23/07/18 → 27/07/18 |

### Bibliographical note

Publisher Copyright:© 2018 Association for Computing Machinery.

## Keywords

- Approximation algorithms
- Distributed network design
- Hardness of approximation
- Spanners

## ASJC Scopus subject areas

- Software
- Hardware and Architecture
- Computer Networks and Communications