摘要(英) |
Abstract
Given a weighted directed graph G = (V,E,c), where c : E -> R+ isan edge length function, a subset X of vertices (terminals), and a root vertex vr, directed Steiner tree problem (DSP) asks for a minimum cost tree which spans paths from root vertex vr to each terminal. DSP is often raised in one-to-many (Multicast) data delivering network to improve the cost of the distribution tree1. Before this article, Charikar et al’s DSP algorithm is well known for IDMR. It achieves an approximation ratio of 1(l−1)k^(1/l) in O(n^lk^{2l-2)logn+m) times for any fixed level l > 1, where l is the level of the tree produced by the algorithm, n is the number of vertices, |V |, and k is the number
of terminals, |X|. Charikar et al’s DSP algorithm is useful to improve for IDMR. However it requires a great amount of computing power. This thesis provides a faster approximation algorithm based on ideas of Charikar et al’s DSP algorithm with better time complexity,
O(P^n_lP^k_l+n^2k+nm), and a better approximation ratio for any level l > 1. |
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