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A threshold of ln n for approximating set cover

1998/07/01 by Uriel Feige · 3,133 citations
Computer Science · Mathematics · #Advanced Graph Theory Research #Algorithm #Approximation algorithm #Cardinality (data modeling) #Combinatorics #Complexity and Algorithms in Graphs #Computational Geometry and Mesh Generation #Computer science #Cover (algebra) #Discrete mathematics #Greedy algorithm #Mathematics #Order (exchange) #Set (abstract data type) #Set cover problem

paper · pdf · doi:10.1145/285055.285059

published in Journal of the ACM 45(4), 634-652 (Association for Computing Machinery)

openalex publication_date 1998/07/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05

Abstract

Given a collection ℱ of subsets of S = 1,…, n , set cover is the problem of selecting as few as possible subsets from ℱ such that their union covers S, , and max k-cover is the problem of selecting k subsets from ℱ such that their union has maximum cardinality. Both these problems are NP-hard. We prove that (1 - o (1)) ln n is a threshold below which set cover cannot be approximated efficiently, unless NP has slightly superpolynomial time algorithms. This closes the gap (up to low-order terms) between the ratio of approximation achievable by the greedy alogorithm (which is (1 - o (1)) ln n), and provious results of Lund and Yanakakis, that showed hardness of approximation within a ratio of (log 2 n ) / 2 ≃0.72 ln n . For max k -cover, we show an approximation threshold of (1 - 1/ e )(up to low-order terms), under assumption that P ≠ NP .

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