Matthias Hein Olivier Bousquet Maximal Margin Classification for Metric Spaces. 72-86 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2777&spage=72 2003 conf/colt/2003 COLT db/conf/colt/colt2003.html#HeinB03