José R. Correa Martin Skutella José Verschae The Power of Preemption on Unrelated Machines and Applications to Scheduling Orders. 84-97 2009 APPROX-RANDOM http://dx.doi.org/10.1007/978-3-642-03685-9_7 conf/approx/2009 db/conf/approx/approx2009.html#CorreaSV09