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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 10 |
| 4 | 54 |
| 5 | 336 |
| 6 | 2400 |
| 7 | 19440 |
| 8 | 176400 |
| 9 | 1774080 |
| 10 | 19595520 |
| 11 | 235872000 |
| 12 | 3073593600 |
| 13 | 43110144000 |
| 14 | 647610163200 |
| 15 | 10374216652800 |
| 16 | 176536039680000 |
| 17 | 3180264062976000 |
| 18 | 60466862776320000 |
| 19 | 1210048630382592000 |
| 20 | 25423825985445888000 |
| 21 | 559567461880627200000 |
| 22 | 12874917427270778880000 |
[1,1,2,10,54,336,2400,19440,176400,1774080, 19595520,235872000,3073593600,43110144000, 647610163200,10374216652800,176536039680000, 3180264062976000,60466862776320000, 1210048630382592000,25423825985445888000, 559567461880627200000,12874917427270778880000] |