Maurice H. ter Beek Erzsébet Csuhaj-Varjú Victor Mitrana 141-156 2004 81 Int. J. Comput. Math. 2 http://taylorandfrancis.metapress.com/openurl.asp?genre=article&issn=0020-7160&volume=81&issue=2&spage=141 db/journals/ijcm/ijcm81.html#BeekCM04