Zulfiqar Habib Manabu Sakai G2 Planar Cubic Transition between Two Circles. 957-965 2003 80 Int. J. Comput. Math. 8 http://taylorandfrancis.metapress.com/openurl.asp?genre=article&issn=0020-7160&volume=80&issue=8&spage=957 db/journals/ijcm/ijcm80.html#HabibS03