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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 12 |
| 3 | 41 |
| 4 | 159 |
| 5 | 685 |
| 6 | 3232 |
| 7 | 16497 |
| 8 | 90075 |
| 9 | 520248 |
| 10 | 3143900 |
| 11 | 19678699 |
| 12 | 126480319 |
| 13 | 828889655 |
| 14 | 5508880129 |
| 15 | 36980189248 |
| 16 | 249999755818 |
| 17 | 1698491520413 |
| 18 | 11579658486155 |
| 19 | 79137696973965 |
| 20 | 541762392142995 |
| 21 | 3713215153801152 |
[4,12,41,159,685,3232,16497,90075,520248,3143900, 19678699,126480319,828889655,5508880129, 36980189248,249999755818,1698491520413, 11579658486155,79137696973965,541762392142995, 3713215153801152] |