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| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 4 |
| 3 | 21 |
| 4 | 330 |
| 5 | 14610 |
| 6 | 1615666 |
| 7 | 360541154 |
| 8 | 138557401940 |
| 9 | 84078437148114 |
[2,4,21,330,14610,1615666,360541154,138557401940, 84078437148114] |