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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 12 |
| 5 | 68 |
| 6 | 360 |
| 7 | 2384 |
| 8 | 20160 |
| 9 | 185408 |
| 10 | 1814400 |
| 11 | 19781504 |
| 12 | 239500800 |
| 13 | 3124694528 |
| 14 | 43589145600 |
| 15 | 652885305344 |
| 16 | 10461394944000 |
| 17 | 177948646719488 |
| 18 | 3201186852864000 |
| 19 | 60808005761859584 |
| 20 | 1216451004088320000 |
| 21 | 25547946834881282048 |
| 22 | 562000363888803840000 |
[1,1,1,2,12,68,360,2384,20160,185408,1814400, 19781504,239500800,3124694528,43589145600, 652885305344,10461394944000,177948646719488, 3201186852864000,60808005761859584, 1216451004088320000,25547946834881282048, 562000363888803840000] |