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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 24 |
| 4 | 119 |
| 5 | 694 |
| 6 | 4582 |
| 7 | 33324 |
| 8 | 261808 |
| 9 | 2190688 |
| 10 | 19318688 |
| 11 | 178108704 |
[1,2,6,24,119,694,4582,33324,261808,2190688, 19318688,178108704] |