Ingo Demgensky Hartmut Noltemeier Hans-Christoph Wirth Optimizing cost flows by edge cost and capacity upgrade. 407-423 2004 2 J. Discrete Algorithms 4 http://dx.doi.org/10.1016/j.jda.2004.04.003 db/journals/jda/jda2.html#DemgenskyNW04