&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Bulletin of the American Mathematical Society. 1965.<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Vol. 71 No. 6 : Vol. 71 No. 6. &mdash; 1965. &mdash; ISSN 0002-9904.<br><br><br><br>[ SYS NUM: 16076 ]