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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 24 |
| 5 | 240 |
| 6 | 1560 |
| 7 | 8400 |
| 8 | 81144 |
| 9 | 1638000 |
| 10 | 31058520 |
| 11 | 482499600 |
| 12 | 6905646264 |
| 13 | 114015261360 |
| 14 | 2456232531480 |
| 15 | 59734751403600 |
| 16 | 1427946773067384 |
| 17 | 33377481440110320 |
| 18 | 818549745973204440 |
| 19 | 22338800420915168400 |
| 20 | 667566534457962216504 |
| 21 | 20735588176755396824880 |
[1,0,0,0,24,240,1560,8400,81144,1638000,31058520, 482499600,6905646264,114015261360,2456232531480, 59734751403600,1427946773067384,33377481440110320, 818549745973204440,22338800420915168400, 667566534457962216504,20735588176755396824880] |