polylogarithmic inapproximability
We provide the first hardness result of a polylogarithmic approximation ratio for a natural NP-hard optimization problem. We show that for every fixed ?>0 , the GROUP-STEINER-TREE problem admits no efficient log 2-? k approximation, where k denotes the number of groups (or, alternatively, the input size), unless NP has quasi polynomial Las-Vegas algorithms. This hardness result holds even for input graphs which are Hierarchically Well-Separated Trees , introduced by Bartal [FOCS, 1996]. For these trees (and also for general trees), our bound is nearly tight with the log-squared approximation currently known. Our results imply that for every fixed ?>0 , the DIRECTED-STEINER TREE problem admits no log 2-? n --approximation, where n is the number of vertices in the graph, under the same complexity assumption.
polylogarithmic approximation integrality ratio hardness of approximation approximation algorithms steiner tree