Thang Nguyen Bui Gnanasekaran Sundarraj Ant System for the k-Cardinality Tree Problem. 36-47 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3102&spage=36 2004 conf/gecco/2004-1 GECCO (1) db/conf/gecco/gecco2004-1.html#BuiS04