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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 3 |
| 3 | 13 |
| 4 | 61 |
| 5 | 321 |
| 6 | 1951 |
| 7 | 13693 |
| 8 | 109593 |
| 9 | 986401 |
| 10 | 9864091 |
| 11 | 108505101 |
| 12 | 1302061333 |
| 13 | 16926797473 |
| 14 | 236975164791 |
| 15 | 3554627472061 |
| 16 | 56874039553201 |
| 17 | 966858672404673 |
| 18 | 17403456103284403 |
| 19 | 330665665962403981 |
| 20 | 6613313319248079981 |
| 21 | 138879579704209680001 |
[1,1,3,13,61,321,1951,13693,109593,986401,9864091, 108505101,1302061333,16926797473,236975164791, 3554627472061,56874039553201,966858672404673, 17403456103284403,330665665962403981, 6613313319248079981,138879579704209680001] |