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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 21 |
| 3 | 128 |
| 4 | 851 |
| 5 | 5984 |
| 6 | 43759 |
| 7 | 329396 |
| 8 | 2535406 |
| 9 | 19863592 |
| 10 | 157874971 |
| 11 | 1269833668 |
| 12 | 10316765299 |
| 13 | 84540929568 |
| 14 | 697928139977 |
| 15 | 5799156785376 |
| 16 | 48461097907978 |
| 17 | 407020852551016 |
| 18 | 3434002483872566 |
| 19 | 29090171931564848 |
| 20 | 247333930963224287 |
| 21 | 2109921586071433064 |
[1,4,21,128,851,5984,43759,329396,2535406, 19863592,157874971,1269833668,10316765299, 84540929568,697928139977,5799156785376, 48461097907978,407020852551016,3434002483872566, 29090171931564848,247333930963224287, 2109921586071433064] |