&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Bulletin of the American Mathematical Society. 1971.<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Vol. 77 No. 4 : Vol. 77 No. 4. &mdash; 1971. &mdash; ISSN 0002-9904.<br><br><br><br>[ SYS NUM: 16111 ]