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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 21 |
| 2 | 135 |
| 3 | 693 |
| 4 | 4159 |
| 5 | 25291 |
| 6 | 157635 |
| 7 | 995659 |
| 8 | 6357119 |
| 9 | 40921479 |
| 10 | 265145505 |
| 11 | 1727138841 |
| 12 | 11300204035 |
| 13 | 74208972755 |
| 14 | 488874737353 |
| 15 | 3229363648333 |
| 16 | 21382430152111 |
| 17 | 141869013922279 |
| 18 | 942972599847861 |
| 19 | 6277675948529895 |
[1,21,135,693,4159,25291,157635,995659,6357119, 40921479,265145505,1727138841,11300204035, 74208972755,488874737353,3229363648333, 21382430152111,141869013922279,942972599847861, 6277675948529895] |