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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 28 |
| 4 | 244 |
| 5 | 2544 |
| 6 | 35600 |
| 7 | 659632 |
| 8 | 15106128 |
| 9 | 425802176 |
| 10 | 14409526080 |
| 11 | 577386122880 |
[1,2,6,28,244,2544,35600,659632,15106128, 425802176,14409526080,577386122880] |