| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 6 |
| 3 | 27 |
| 4 | 125 |
| 5 | 635 |
| 6 | 3488 |
| 7 | 20425 |
| 8 | 126817 |
| 9 | 831915 |
| 10 | 5744784 |
| 11 | 41618459 |
| 12 | 315388311 |
| 13 | 2493721645 |
| 14 | 20526285716 |
| 15 | 175529425815 |
| 16 | 1556577220651 |
| 17 | 14290644428279 |
| 18 | 135624265589086 |
| 19 | 1328702240382589 |
| 20 | 13420603191219111 |
| 21 | 139592874355534071 |
[1,1,6,27,125,635,3488,20425,126817,831915, 5744784,41618459,315388311,2493721645,20526285716, 175529425815,1556577220651,14290644428279, 135624265589086,1328702240382589, 13420603191219111,139592874355534071] |