| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 24 |
| 4 | 172 |
| 5 | 1420 |
| 6 | 13968 |
| 7 | 154336 |
| 8 | 1914288 |
| 9 | 26108208 |
| 10 | 388596960 |
| 11 | 6251899104 |
| 12 | 108088087776 |
| 13 | 1995840455232 |
| 14 | 39183950494752 |
| 15 | 814399382073120 |
| 16 | 17856182764554240 |
| 17 | 411671923447488768 |
| 18 | 9952212794293198080 |
| 19 | 251646630845685827328 |
| 20 | 6640389412581544588800 |
[1,1,4,24,172,1420,13968,154336,1914288,26108208, 388596960,6251899104,108088087776,1995840455232, 39183950494752,814399382073120,17856182764554240, 411671923447488768,9952212794293198080, 251646630845685827328,6640389412581544588800] |