Farhad Shahrokhi Ondrej Sýkora László A. Székely Imrich Vrto Bounds and Methods for k-Planar Crossing Numbers. 37-46 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2912&spage=37 2003 conf/gd/2003 Graph Drawing db/conf/gd/gd2003.html#ShahrokhiSSV03