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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 672 |
| 4 | 13440 |
| 5 | 58705920 |
| 6 | 234823680 |
| 7 | 11243357798400 |
| 8 | 494707743129600 |
| 9 | 4321766843980185600 |
| 10 | 86435336879603712000 |
| 11 | 450155234468976132096000 |
[1,4,672,13440,58705920,234823680,11243357798400, 494707743129600,4321766843980185600, 86435336879603712000,450155234468976132096000] |