Ashkan Aazami Joseph Cheriyan Krishnam Raju Jampani Approximation Algorithms and Hardness Results for Packing Element-Disjoint Steiner Trees in Planar Graphs. 1-14 2009 APPROX-RANDOM http://dx.doi.org/10.1007/978-3-642-03685-9_1 conf/approx/2009 db/conf/approx/approx2009.html#AazamiCJ09