&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Bulletin (New Series) of the American Mathematical Society. 1990.<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Vol. 23 No. 2 : Vol. 23 No. 2. &mdash; 1990. &mdash; ISSN 0273-0979.<br><br><br><br>[ SYS NUM: 15715 ]