| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 100 |
| 5 | 660 |
| 6 | 5038 |
| 7 | 43624 |
| 8 | 422252 |
| 9 | 4516380 |
| 10 | 52885644 |
| 11 | 672781824 |
| 12 | 9238314358 |
| 13 | 136175455234 |
| 14 | 2144494356834 |
| 15 | 35930786795040 |
| 16 | 638168940129732 |
| 17 | 11976278012219556 |
| 18 | 236791150694618872 |
| 19 | 4919643784275283480 |
| 20 | 107152493449339765396 |
| 21 | 2441410548192907949196 |
[1,1,4,18,100,660,5038,43624,422252,4516380, 52885644,672781824,9238314358,136175455234, 2144494356834,35930786795040,638168940129732, 11976278012219556,236791150694618872, 4919643784275283480,107152493449339765396, 2441410548192907949196] |