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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 15 |
| 3 | 511 |
| 4 | 65089 |
| 5 | 33296833 |
| 6 | 68184176641 |
| 7 | 558512367132769 |
| 8 | 18299400300282885215 |
[1,15,511,65089,33296833,68184176641, 558512367132769,18299400300282885215] |