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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 2 |
| 3 | 15 |
| 4 | 231 |
| 5 | 10649 |
| 6 | 1209990 |
| 7 | 352842084 |
| 8 | 271939715540 |
| 9 | 541622294917014 |
[1,2,15,231,10649,1209990,352842084,271939715540, 541622294917014] |