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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 4 |
| 5 | 20 |
| 6 | 105 |
| 7 | 840 |
| 8 | 3960 |
| 9 | 51480 |
| 10 | 675675 |
| 11 | 10810800 |
| 12 | 139675536 |
| 13 | 2793510720 |
| 14 | 58663725120 |
| 15 | 1799020903680 |
| 16 | 26985313555200 |
| 17 | 782574093100800 |
| 18 | 25992639520848000 |
| 19 | 857757104187984000 |
| 20 | 30021498646579440000 |
| 21 | 1563341744336692320000 |
| 22 | 64179292662243158400000 |
[1,1,1,2,4,20,105,840,3960,51480,675675,10810800, 139675536,2793510720,58663725120,1799020903680, 26985313555200,782574093100800,25992639520848000, 857757104187984000,30021498646579440000, 1563341744336692320000,64179292662243158400000] |