Fabián A. Chudak Tim Roughgarden David P. Williamson Approximate k-MSTs and k-Steiner Trees via the Primal-Dual Method and Lagrangean Relaxation. 60-70 2001 conf/ipco/2001 IPCO http://link.springer.de/link/service/series/0558/bibs/2081/20810060.htm db/conf/ipco/ipco2001.html#ChudakRW01