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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 18 |
| 3 | 172 |
| 4 | 1881 |
| 5 | 25644 |
| 6 | 411488 |
| 7 | 7581792 |
| 8 | 157296141 |
| 9 | 3630709734 |
| 10 | 92225760033 |
| 11 | 2556393882912 |
| 12 | 76774488792326 |
| 13 | 2483261807354874 |
[1,4,18,172,1881,25644,411488,7581792,157296141, 3630709734,92225760033,2556393882912, 76774488792326,2483261807354874] |