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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 3 |
| 4 | 54 |
| 5 | 250 |
| 6 | 1950 |
| 7 | 10206 |
| 8 | 64288 |
| 9 | 350064 |
| 10 | 2065500 |
| 11 | 11509300 |
| 12 | 66905256 |
| 13 | 380767608 |
| 14 | 2226036904 |
| 15 | 12949377000 |
| 16 | 76842172800 |
| 17 | 457297336576 |
| 18 | 2766381692688 |
| 19 | 16849247813424 |
| 20 | 104116268476000 |
| 21 | 649043824951200 |
[0,1,3,54,250,1950,10206,64288,350064,2065500, 11509300,66905256,380767608,2226036904, 12949377000,76842172800,457297336576, 2766381692688,16849247813424,104116268476000, 649043824951200] |