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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 1 |
| 3 | 4 |
| 4 | 12 |
| 5 | 52 |
| 6 | 360 |
| 7 | 2656 |
| 8 | 20160 |
| 9 | 177472 |
| 10 | 1814400 |
| 11 | 20135296 |
| 12 | 239500800 |
| 13 | 3102326272 |
| 14 | 43589145600 |
| 15 | 654789062656 |
| 16 | 10461394944000 |
| 17 | 177738781376512 |
| 18 | 3201186852864000 |
| 19 | 60837094646972416 |
| 20 | 1216451004088320000 |
| 21 | 25542995336828157952 |
| 22 | 562000363888803840000 |
[0,0,1,4,12,52,360,2656,20160,177472,1814400, 20135296,239500800,3102326272,43589145600, 654789062656,10461394944000,177738781376512, 3201186852864000,60837094646972416, 1216451004088320000,25542995336828157952, 562000363888803840000] |