Tetsuo Asano Peter Braß Shinji Sasahara Disc Covering Problem with Application to Digital Halftoning. 11-21 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3045&spage=11 2004 conf/iccsa/2004-3 ICCSA (3) db/conf/iccsa/iccsa2004-3.html#AsanoBS04