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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 6 |
| 3 | 30 |
| 4 | 264 |
| 5 | 2520 |
| 6 | 30960 |
| 7 | 428400 |
| 8 | 6894720 |
| 9 | 123742080 |
| 10 | 2478470400 |
| 11 | 54486432000 |
| 12 | 1308153369600 |
| 13 | 34005760588800 |
| 14 | 952248474777600 |
| 15 | 28566146568960000 |
| 16 | 914137612996608000 |
| 17 | 31080323154456576000 |
| 18 | 1118898035934142464000 |
| 19 | 42518003720397004800000 |
[1,1,6,30,264,2520,30960,428400,6894720,123742080, 2478470400,54486432000,1308153369600, 34005760588800,952248474777600,28566146568960000, 914137612996608000,31080323154456576000, 1118898035934142464000,42518003720397004800000] |