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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 16 |
| 3 | 96 |
| 4 | 640 |
| 5 | 5280 |
| 6 | 27648 |
| 7 | 340704 |
| 8 | 1966080 |
| 9 | 15676416 |
| 10 | 124608000 |
| 11 | 1341648000 |
| 12 | 7166361600 |
| 13 | 97763702400 |
| 14 | 629315721216 |
| 15 | 4680529920000 |
| 16 | 42316647628800 |
| 17 | 483414202656000 |
| 18 | 2396062681399296 |
| 19 | 35513562609100800 |
| 20 | 211942058803200000 |
| 21 | 2006922093666287616 |
| 22 | 16837843397760000000 |
[1,4,16,96,640,5280,27648,340704,1966080,15676416, 124608000,1341648000,7166361600,97763702400, 629315721216,4680529920000,42316647628800, 483414202656000,2396062681399296, 35513562609100800,211942058803200000, 2006922093666287616,16837843397760000000] |