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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 24 |
| 4 | 168 |
| 5 | 1560 |
| 6 | 17280 |
| 7 | 221760 |
| 8 | 3265920 |
| 9 | 54069120 |
| 10 | 994291200 |
| 11 | 20118067200 |
| 12 | 444034483200 |
| 13 | 10617070464000 |
| 14 | 273391121203200 |
| 15 | 7542665754624000 |
| 16 | 221969877921792000 |
| 17 | 6940528784437248000 |
| 18 | 229781192298577920000 |
| 19 | 8030036368187817984000 |
| 20 | 295390797322766745600000 |
[1,1,4,24,168,1560,17280,221760,3265920,54069120, 994291200,20118067200,444034483200,10617070464000, 273391121203200,7542665754624000, 221969877921792000,6940528784437248000, 229781192298577920000,8030036368187817984000, 295390797322766745600000] |