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| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 21 |
| 3 | 137 |
| 4 | 735 |
| 5 | 3557 |
| 6 | 16191 |
| 7 | 70877 |
| 8 | 302295 |
| 9 | 1266437 |
| 10 | 5239311 |
| 11 | 21481517 |
| 12 | 87506055 |
| 13 | 354778517 |
| 14 | 1433405631 |
| 15 | 5776554557 |
| 16 | 23235129015 |
| 17 | 93327477797 |
| 18 | 374471255151 |
| 19 | 1501369969997 |
| 20 | 6015936563175 |
| 21 | 24095119972277 |
| 22 | 96474608387871 |
[2,21,137,735,3557,16191,70877,302295,1266437, 5239311,21481517,87506055,354778517,1433405631, 5776554557,23235129015,93327477797,374471255151, 1501369969997,6015936563175,24095119972277, 96474608387871] |