&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Bulletin of the American Mathematical Society. 1973.<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Vol. 79 No. 4 : Vol. 79 No. 4. &mdash; 1973. &mdash; ISSN 0002-9904.<br><br><br><br>[ SYS NUM: 16123 ]