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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 3 |
| 3 | 15 |
| 4 | 63 |
| 5 | 255 |
| 6 | 891 |
| 7 | 4095 |
| 8 | 262143 |
| 9 | 1048575 |
| 10 | 16777215 |
| 11 | 68719476735 |
[1,3,15,63,255,891,4095,262143,1048575,16777215, 68719476735] |