Free multiflows in bidirected and skew-symmetric graphs

  • Authors:
  • Maxim A. Babenko;Alexander V. Karzanov

  • Affiliations:
  • Department of Mechanics and Mathematics, Moscow State University, Vorob'yovy Gory, 119899 Moscow, Russia;Institute for System Analysis, 9, Prospect 60 Let Oktyabrya, 117312 Moscow, Russia

  • Venue:
  • Discrete Applied Mathematics
  • Year:
  • 2007

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Abstract

A graph (digraph) G=(V,E) with a set T@?V of terminals is called inner Eulerian if each nonterminal node v has even degree (resp. the numbers of edges entering and leaving v are equal). Cherkassky and Lovasz, independently, showed that the maximum number of pairwise edge-disjoint T-paths in an inner Eulerian graph G is equal to 12@?"s"@?"T@l(s), where @l(s) is the minimum number of edges whose removal disconnects s and T-{s}. A similar relation for inner Eulerian digraphs was established by Lomonosov. Considering undirected and directed networks with ''inner Eulerian'' edge capacities, Ibaraki, Karzanov, and Nagamochi showed that the problem of finding a maximum integer multiflow (where partial flows connect arbitrary pairs of distinct terminals) is reduced to O(logT) maximum flow computations and to a number of flow decompositions. In this paper we extend the above max-min relation to inner Eulerian bidirected graphs and inner Eulerian skew-symmetric graphs and develop an algorithm of complexity O(VElogTlog(2+V^2/E)) for the corresponding capacitated cases. In particular, this improves the best known bound for digraphs. Our algorithm uses a fast procedure for decomposing a flow with O(1) sources and sinks in a digraph into the sum of one-source-one-sink flows.