Christoph Eckert Jens Gottlieb Direct Representation and Variation Operators for the Fixed Charge Transportation Problem. 77-87 2002 conf/ppsn/2002 PPSN http://link.springer.de/link/service/series/0558/bibs/2439/24390077.htm db/conf/ppsn/ppsn2002.html#EckertG02