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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 30 |
| 4 | 288 |
| 5 | 3500 |
| 6 | 51882 |
| 7 | 908705 |
| 8 | 18376192 |
| 9 | 421518897 |
| 10 | 10815546010 |
| 11 | 306954846231 |
| 12 | 9547629128208 |
| 13 | 322979502072591 |
| 14 | 11805623386524688 |
| 15 | 463679308850798265 |
| 16 | 19474458473055138816 |
| 17 | 870962008703995217038 |
| 18 | 41324081662873427484240 |
| 19 | 2073203796753598883831150 |
| 20 | 109655938011610286565760400 |
[1,1,4,30,288,3500,51882,908705,18376192, 421518897,10815546010,306954846231,9547629128208, 322979502072591,11805623386524688, 463679308850798265,19474458473055138816, 870962008703995217038,41324081662873427484240, 2073203796753598883831150, 109655938011610286565760400] |