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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 22 |
| 4 | 140 |
| 5 | 969 |
| 6 | 7084 |
| 7 | 53820 |
| 8 | 420732 |
| 9 | 3362260 |
| 10 | 27343888 |
| 11 | 225568798 |
| 12 | 1882933364 |
| 13 | 15875338990 |
| 14 | 134993766600 |
| 15 | 1156393243320 |
| 16 | 9969937491420 |
| 17 | 86445222719724 |
| 18 | 753310723010608 |
| 19 | 6594154339031800 |
| 20 | 57956002331347120 |
| 21 | 511238042454541545 |
[1,1,4,22,140,969,7084,53820,420732,3362260, 27343888,225568798,1882933364,15875338990, 134993766600,1156393243320,9969937491420, 86445222719724,753310723010608,6594154339031800, 57956002331347120,511238042454541545] |