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| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 4 |
| 3 | 20 |
| 4 | 1540 |
| 5 | 609896980 |
| 6 | 37811003218473324699257860 |
| 7 | 9009555207802177724984164589516456320805205201729086740415363658290866918420 |
[2,4,20,1540,609896980,37811003218473324699257860, 90095552078021777249841645895164563208052052017290\ 86740415363658290866918420] |