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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 2 |
| 3 | 6 |
| 4 | 22 |
| 5 | 100 |
| 6 | 308 |
| 7 | 1512 |
| 8 | 51144 |
| 9 | 869472 |
| 10 | 12648528 |
| 11 | 168624720 |
| 12 | 2029008720 |
[0,0,2,6,22,100,308,1512,51144,869472,12648528, 168624720,2029008720] |