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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 0 |
| 3 | 3 |
| 4 | 6 |
| 5 | 20 |
| 6 | 180 |
| 7 | 1134 |
| 8 | 7980 |
| 9 | 78840 |
| 10 | 798840 |
| 11 | 8620920 |
| 12 | 107668440 |
| 13 | 1449377280 |
| 14 | 20755871136 |
| 15 | 323448048000 |
| 16 | 5398086002400 |
| 17 | 95487623038080 |
| 18 | 1796842848654720 |
| 19 | 35808112038746880 |
| 20 | 751616958775939200 |
| 21 | 16600116241063514880 |
| 22 | 384905905873078867200 |
[0,0,0,3,6,20,180,1134,7980,78840,798840,8620920, 107668440,1449377280,20755871136,323448048000, 5398086002400,95487623038080,1796842848654720, 35808112038746880,751616958775939200, 16600116241063514880,384905905873078867200] |