Combinatorial logarithmic approximation algorithm for directed telephone broadcast problem

• Authors: Michael Elkin;Guy Kortsarz
• Venue: STOC '02 Proceedings of the thiry-fourth annual ACM symposium on Theory of computing
• Year: 2002

(MATH) Consider a synchronous network of processors, modeled by directed or undirected graph G = (V,E), in which on each round every processor is allowed to choose one of its neighbors and to send him a message. Given a processor s &egr; V, and a subset T ⊆ V of processors, the telephone multicast problem requires to compute the shortest schedule (in terms of the number of rounds) that delivers a message from s to all the processors of T. The particular case T = V is called telephone broadcast problem.These problems have multiple applications in distributed computing. Several approximation algorithms with polylogarithmic ratio, including one with logarithmic ratio, for the undirected variants of these problems are known. However, all these algorithms involve solving large linear programs. Devising a polylogarithmic approximation algorithm for the directed variants of these problems is anopen problem, posed in [15].We devise a combinatorial logarithmic approximation algorithm for these problems, that applies also for the directed broadcast problem. Our algorithm has significantly smaller running time, and seems to reveal more information about the combinatorial structure of the solution, than the previous algorithms, that are based on linear programming.(MATH) We also improve the lower bounds on the approximation threshold of these problems. Both problems are known to be 3/2-inapproximable. For the undirected (resp., directed) broadcast problem we show that it is NP-hard (resp., impossible unless $NP ⊇ DTIME(nO(log n))) to approximate it within a ratio of 3 —&egr; for any &egr; ρ 0 (resp., ω(\sqrt log n)).Finally, we study the radio broadcast problem. Its setting is similar to the telephone broadcast problem, but in every round every processor may either send a message to all its neighbors or may not send it at all. A processor is informed in a certain round if and only if it receives a message from precisely one neighbor.(MATH) This problem was known to admit O(log2 n)-approximation algorithm, but no hardness of approximation was known. In this paper we show that the problem is ω(log n)-inapproximable unless NP ⊆ BPTIME(nlog log n}).