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| n | a(n) |
|---|---|
| 1 | 6 |
| 2 | 21 |
| 3 | 54 |
| 4 | 75 |
| 5 | 441 |
| 6 | 1071 |
| 7 | 4191 |
| 8 | 9315 |
| 9 | 58311 |
| 10 | 4197375 |
| 11 | 7685151 |
| 12 | 36997695 |
| 13 | 268460031 |
| 14 | 1073790975 |
| 15 | 17180065791 |
| 16 | 13517087570210304 |
| 17 | 18014398710808575 |
| 18 | 288230376957018111 |
[6,21,54,75,441,1071,4191,9315,58311,4197375, 7685151,36997695,268460031,1073790975,17180065791, 13517087570210304,18014398710808575, 288230376957018111] |