Approximating the Minimum Equivalent Digraph

  • Authors:
  • Samir Khuller;Balaji Raghavachari;Neal Young

  • Affiliations:
  • -;-;-

  • Venue:
  • SIAM Journal on Computing
  • Year:
  • 1995

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Abstract

The MEG (minimum equivalent graph) problem is the following: "Given a directed graph, find a smallest subset of the edges that maintains all reachability relations between nodes." This problem is NP-hard; this paper gives an approximation algorithm achieving a performance guarantee of about 1.64 in polynomial time. The algorithm achieves a performance guarantee of 1.75 in the time required for transitive closure. The heart of the MEG problem is the minimum SCSS (strongly connected spanning subgraph) problem --- the MEG problem restricted to strongly connected digraphs. For the minimum SCSS problem, the paper gives a practical, nearly linear-time implementation achieving a performance guarantee of 1.75. The algorithm and its analysis are based on the simple idea of contracting long cycles. The analysis applies directly to $2$-\Exchange, a general "local improvement" algorithm, showing that its performance guarantee is 1.75.