&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Bulletin of the American Mathematical Society. 1968.<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Vol. 74 No. 6 : Vol. 74 No. 6. &mdash; 1968. &mdash; ISSN 0002-9904.<br><br><br><br>[ SYS NUM: 16095 ]