Chandra Chekuri Alina Ene Nitish Korula Unsplittable Flow in Paths and Trees and Column-Restricted Packing Integer Programs. 42-55 2009 APPROX-RANDOM http://dx.doi.org/10.1007/978-3-642-03685-9_4 conf/approx/2009 db/conf/approx/approx2009.html#ChekuriEK09