The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 116 |
| 5 | 728 |
| 6 | 4818 |
| 7 | 33100 |
| 8 | 233824 |
| 9 | 1687764 |
| 10 | 12393520 |
| 11 | 92291681 |
| 12 | 695325926 |
| 13 | 5290359124 |
| 14 | 40591599128 |
| 15 | 313725215636 |
| 16 | 2440203573816 |
| 17 | 19087022233906 |
| 18 | 150042056387660 |
| 19 | 1184734863936672 |
| 20 | 9392213303130904 |
| 21 | 74728563957003952 |
| 22 | 596531545003840160 |
[1,1,4,20,116,728,4818,33100,233824,1687764, 12393520,92291681,695325926,5290359124, 40591599128,313725215636,2440203573816, 19087022233906,150042056387660,1184734863936672, 9392213303130904,74728563957003952, 596531545003840160] |