What Would Edmonds Do? Augmenting Paths and Witnesses for Degree-Bounded MSTs

  • Kamalika Chaudhuri ,
  • Satish Rao ,
  • Samantha Riesenfeld ,
  • Kunal Talwar

8th International Workshop on Approximation Algorithms for Combinatorial Optimization Problems (APPROX 2005) and 9th International Workshop on Randomization and Computation (RANDOM 2005) |

Published by Springer Verlag

Publication

Given a graph and degree upper bounds on vertices, the BDMST problem requires us to find a minimum cost spanning tree respecting the given degree bounds. This problem generalizes the Travelling Salesman Path Problem (TSPP), even in unweighted graphs, and so we expect that it is necessary to relax the degree constraints to get efficient algorithms. Könemann and Ravi (Proceedings of the Thirty Second Annual ACM Symposium on Theory of Computing, pp. 537–546, 2000; Proceedings of the Thirty-Fifth ACM Symposium on Theory of Computing, pp. 389–395, 2003) give bicriteria approximation algorithms for the problem using local search techniques of Fischer (Technical Report 14853, Cornell University, 1993). Their algorithms find solutions which make a tradeoff of the approximation factor for the cost of the resulting tree against the factor by which degree constraints are violated. In particular, they give an algorithm which, for a graph with a spanning tree of cost C and degree B, and for parameters b,w >1, produces a tree whose cost is at most wC and whose degree is at most w w−1bB+logb n.

A primary contribution of Könemann and Ravi is to use a Lagrangean relaxation to formally relate the BDMST problem to what we call the MDMST problem, which is the problem of finding an MST of minimum degree in a graph. In their solution to the MDMST problem, they make central use of a local-search approximation algorithm of Fischer.