Length-bounded cuts and flows

  • Authors:
  • Georg Baier;Thomas Erlebach;Alexander Hall;Ekkehard Köhler;Petr Kolman;Ondřej Pangrác;Heiko Schilling;Martin Skutella

  • Affiliations:
  • Siemens, Munich, Germany;University of Leicester, Leicester, U.K.;Google, Zürich, Switzerland;BTU Cottbus, Cottbus, Germany;Charles University in Prague, Prague, Czech Republic;Charles University in Prague, Prague, Czech Republic;TomTom, DK Amsterdam, The Netherlands;TU Berlin, Berlin, Germany

  • Venue:
  • ACM Transactions on Algorithms (TALG)
  • Year:
  • 2010

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Abstract

For a given number L, an L-length-bounded edge-cut (node-cut, respectively) in a graph G with source s and sink t is a set C of edges (nodes, respectively) such that no s-t-path of length at most L remains in the graph after removing the edges (nodes, respectively) in C. An L-length-bounded flow is a flow that can be decomposed into flow paths of length at most L. In contrast to classical flow theory, we describe instances for which the minimum L-length-bounded edge-cut (node-cut, respectively) is Θ(n2/3)-times (Θ(&sqrt;n)-times, respectively) larger than the maximum L-length-bounded flow, where n denotes the number of nodes; this is the worst case. We show that the minimum length-bounded cut problem is NP-hard to approximate within a factor of 1.1377 for L≥ 5 in the case of node-cuts and for L≥ 4 in the case of edge-cuts. We also describe algorithms with approximation ratio O(min{L,n/L}) ⊆ O&sqrt;n in the node case and O(min {L,n2/L2,&sqrt;m} ⊆ O2/3 in the edge case, where m denotes the number of edges. Concerning L-length-bounded flows, we show that in graphs with unit-capacities and general edge lengths it is NP-complete to decide whether there is a fractional length-bounded flow of a given value. We analyze the structure of optimal solutions and present further complexity results.