Deterministic pivoting algorithms for constrained ranking and clustering problems

  • Authors:
  • Anke van Zuylen;Rajneesh Hegde;Kamal Jain;David P. Williamson

  • Affiliations:
  • Cornell University, Ithaca, NY;Microsoft Corporation, Redmond, WA;Microsoft Research, Redmond, WA;Cornell University, Ithaca, NY

  • Venue:
  • SODA '07 Proceedings of the eighteenth annual ACM-SIAM symposium on Discrete algorithms
  • Year:
  • 2007

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Abstract

We introduce new problems of finding minimum-cost rankings and clusterings which must be consistent with certain constraints (e.g. an input partial order in the case of ranking problems); we give deterministic approximation algorithms for these problems. Randomized approximation algorithms for unconstrained versions of these problems were given by Ailon, Charikar, and Newman [2] and by Ailon and Charikar [1]. Finding deterministic approximation algorithms for these problems answers an open question of Ailon et al. [2]. In particular, we give deterministic algorithms for constrained weighted feedback arc set in tournaments, constrained correlation clustering, and constrained hierarchical clustering related to finding good ultrametrics. Our algorithms follow the paradigm of Ailon et al. [2] of choosing a particular vertex as a pivot and partitioning the graph according to the pivot; unlike their algorithms, we do not choose the pivot randomly but rather use an LP relaxation to choose a good pivot deterministically. Additionally, the use of the LP relaxation allows us to impose constraints easily and analyze the results. In several cases we are able to find approximation factors for the constrained problems that improve on the factors they obtained for the unconstrained cases. We also give a combinatorial algorithm for constrained weighted feedback arc set in tournaments with weights satisfying probability constraints. This algorithm improves on the best known factor given by deterministic combinatorial algorithms for the unconstrained case.