&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Bulletin of the American Mathematical Society. 1960.<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Vol. 66 No. 4 : Vol. 66 No. 4. &mdash; 1960. &mdash; ISSN 0002-9904.<br><br><br><br>[ SYS NUM: 16044 ]