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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 18 |
| 3 | 172 |
| 4 | 866 |
| 5 | 7326 |
| 6 | 35042 |
| 7 | 291982 |
| 8 | 1387224 |
| 9 | 11520638 |
| 10 | 54650290 |
| 11 | 453546794 |
| 12 | 2150825840 |
| 13 | 17847352698 |
| 14 | 84630769202 |
| 15 | 702236000852 |
| 16 | 3329901981396 |
| 17 | 27630151135038 |
| 18 | 131017792653932 |
| 19 | 1087129854047788 |
| 20 | 5154993778435048 |
[4,18,172,866,7326,35042,291982,1387224,11520638, 54650290,453546794,2150825840,17847352698, 84630769202,702236000852,3329901981396, 27630151135038,131017792653932,1087129854047788, 5154993778435048] |