&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Bulletin of the American Mathematical Society. 1955.<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Vol. 61 No. 4 : Vol. 61 No. 4. &mdash; 1955. &mdash; ISSN 0002-9904.<br><br><br><br>[ SYS NUM: 16013 ]