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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 3 |
| 2 | 8 |
| 3 | 20 |
| 4 | 120 |
| 5 | 432 |
| 6 | 1512 |
| 7 | 5184 |
| 8 | 17496 |
| 9 | 58320 |
| 10 | 192456 |
| 11 | 629856 |
| 12 | 2047032 |
[1,3,8,20,120,432,1512,5184,17496,58320,192456, 629856,2047032] |