Approximation algorithms for certain network improvement problems

Abstract. We study budget constrained network upgrading problems. Such problems aim at nding optimal strategies for improving a network under some cost measure subject to certain budget constraints. Given an edge weighted graph G = (V;E), in the edge based upgrading model, it is assumed that each edge e of the given network also has an associated function c e (t) that speci es the cost of upgrading the edge by an amount t. A reduction strategy species for each edge e the amount by which the length `(e) is to be reduced. In the node based upgrading model, a node v can be upgraded at an expense of c(v). Such an upgrade reduces the delay of each edge incident on v. For a given budget B, the goal is to nd an improvement strategy such that the total cost of reduction is at most the given budget B and the cost of a subgraph (e.g. minimum spanning tree) under the modied edge lengths is the best over all possible strategies which obey the budget constraint. After providing a brief overview of the models and denitions of the various problems considered, we present several new results on the complexity and approximability of network improvement problems