R. Weiskircher, G.W. Klau, I. Ljubic, P. Mutzel, U. Pferschy:
"The fractional prize-collecting Steiner tree problem on trees";
Report for submitted to ESA, Technical Report TR-186-1-03-01; 2003.
http://www.ads.tuwien.ac.at/publications/bib/pdf/klau-03.pdf