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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 24 |
| 5 | 120 |
| 6 | 0 |
| 7 | 2520 |
| 8 | 80640 |
| 9 | 907200 |
| 10 | 4838400 |
| 11 | 79833600 |
| 12 | 2395008000 |
| 13 | 40994553600 |
| 14 | 515804889600 |
| 15 | 10025503488000 |
| 16 | 286642221465600 |
| 17 | 6669139276800000 |
| 18 | 133382785536000000 |
| 19 | 3254006435936256000 |
| 20 | 98762305409703936000 |
| 21 | 2851300331032817664000 |
[1,0,0,0,24,120,0,2520,80640,907200,4838400, 79833600,2395008000,40994553600,515804889600, 10025503488000,286642221465600,6669139276800000, 133382785536000000,3254006435936256000, 98762305409703936000,2851300331032817664000] |