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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 15 |
| 3 | 255 |
| 4 | 3135 |
| 5 | 41055 |
| 6 | 440895 |
| 7 | 10705695 |
| 8 | 242777535 |
| 9 | 4360010655 |
[1,2,15,255,3135,41055,440895,10705695,242777535, 4360010655] |