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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 16 |
| 4 | 78 |
| 5 | 456 |
| 6 | 3120 |
| 7 | 24480 |
| 8 | 216720 |
| 9 | 2136960 |
| 10 | 23224320 |
| 11 | 275788800 |
| 12 | 3552595200 |
| 13 | 49337164800 |
| 14 | 734788454400 |
| 15 | 11681891020800 |
| 16 | 197458829568000 |
| 17 | 3535951491072000 |
| 18 | 66869236482048000 |
| 19 | 1331693730791424000 |
| 20 | 27856727993622528000 |
| 21 | 610658404052336640000 |
[1,4,16,78,456,3120,24480,216720,2136960,23224320, 275788800,3552595200,49337164800,734788454400, 11681891020800,197458829568000,3535951491072000, 66869236482048000,1331693730791424000, 27856727993622528000,610658404052336640000] |