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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 7 |
| 4 | 24 |
| 5 | 180 |
| 6 | 1200 |
| 7 | 10080 |
| 8 | 97440 |
| 9 | 997920 |
| 10 | 12096000 |
| 11 | 149688000 |
| 12 | 2128896000 |
| 13 | 31135104000 |
| 14 | 515804889600 |
| 15 | 8717829120000 |
| 16 | 162151621632000 |
| 17 | 3092504583168000 |
| 18 | 63934815200256000 |
| 19 | 1366817864315904000 |
| 20 | 31019500604252160000 |
| 21 | 733013100880220160000 |
[1,1,1,7,24,180,1200,10080,97440,997920,12096000, 149688000,2128896000,31135104000,515804889600, 8717829120000,162151621632000,3092504583168000, 63934815200256000,1366817864315904000, 31019500604252160000,733013100880220160000] |