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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 4 |
| 3 | 6 |
| 4 | 40 |
| 5 | 180 |
| 6 | 948 |
| 7 | 5880 |
| 8 | 42208 |
| 9 | 344736 |
| 10 | 3158640 |
| 11 | 32091840 |
| 12 | 358107264 |
| 13 | 4353972480 |
| 14 | 57290002560 |
| 15 | 811116633600 |
| 16 | 12295029657600 |
| 17 | 198666240675840 |
| 18 | 3408788192947200 |
| 19 | 61898371424870400 |
| 20 | 1185883197069312000 |
| 21 | 23905764186329088000 |
| 22 | 505813884019270041600 |
[1,0,4,6,40,180,948,5880,42208,344736,3158640, 32091840,358107264,4353972480,57290002560, 811116633600,12295029657600,198666240675840, 3408788192947200,61898371424870400, 1185883197069312000,23905764186329088000, 505813884019270041600] |