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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 23 |
| 3 | 148 |
| 4 | 977 |
| 5 | 6484 |
| 6 | 43079 |
| 7 | 286276 |
| 8 | 1902497 |
| 9 | 12643492 |
| 10 | 84025463 |
| 11 | 558412276 |
| 12 | 3711069041 |
| 13 | 24662841844 |
| 14 | 163903113383 |
| 15 | 1089259330468 |
| 16 | 7238946623297 |
| 17 | 48108239012164 |
| 18 | 319715392487639 |
| 19 | 2124748988791636 |
| 20 | 14120553377944337 |
[1,4,23,148,977,6484,43079,286276,1902497, 12643492,84025463,558412276,3711069041, 24662841844,163903113383,1089259330468, 7238946623297,48108239012164,319715392487639, 2124748988791636,14120553377944337] |