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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 4 |
| 3 | 2 |
| 4 | 28 |
| 5 | 105 |
| 6 | 686 |
| 7 | 4718 |
| 8 | 37864 |
| 9 | 340611 |
| 10 | 3406330 |
| 11 | 37469344 |
| 12 | 449632492 |
| 13 | 5845221941 |
| 14 | 81833107734 |
| 15 | 1227496615330 |
| 16 | 19639945846096 |
| 17 | 333879079382663 |
| 18 | 6009823428889074 |
| 19 | 114186645148891076 |
| 20 | 2283732902977823060 |
| 21 | 47958390962534282489 |
| 22 | 1055084601175754216782 |
[1,0,4,2,28,105,686,4718,37864,340611,3406330, 37469344,449632492,5845221941,81833107734, 1227496615330,19639945846096,333879079382663, 6009823428889074,114186645148891076, 2283732902977823060,47958390962534282489, 1055084601175754216782] |