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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 24 |
| 4 | 252 |
| 5 | 3660 |
| 6 | 73560 |
| 7 | 1921080 |
| 8 | 63411600 |
| 9 | 2574406800 |
| 10 | 125747475840 |
| 11 | 7258472907840 |
| 12 | 487590023511360 |
| 13 | 37629962101892160 |
| 14 | 3299990581104497280 |
| 15 | 325758967714868688000 |
| 16 | 35904380354917794720000 |
| 17 | 4387164775718671231084800 |
| 18 | 590610815931660911894707200 |
| 19 | 87118296156852814044256665600 |
[1,1,4,24,252,3660,73560,1921080,63411600, 2574406800,125747475840,7258472907840, 487590023511360,37629962101892160, 3299990581104497280,325758967714868688000, 35904380354917794720000,4387164775718671231084800, 590610815931660911894707200, 87118296156852814044256665600] |