&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Bulletin of the American Mathematical Society. 1961.<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Vol. 67 No. 6 : Vol. 67 No. 6. &mdash; 1961. &mdash; ISSN 0002-9904.<br><br><br><br>[ SYS NUM: 16052 ]