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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 28 |
| 4 | 264 |
| 5 | 3207 |
| 6 | 47696 |
| 7 | 839412 |
| 8 | 17061280 |
| 9 | 393264145 |
| 10 | 10135913792 |
| 11 | 288839201432 |
| 12 | 9017184333440 |
| 13 | 306045200463519 |
| 14 | 11220008681600256 |
| 15 | 441866073895351128 |
| 16 | 18603606156815235584 |
| 17 | 833860238440653331505 |
| 18 | 39643749441387211150336 |
[1,1,4,28,264,3207,47696,839412,17061280, 393264145,10135913792,288839201432,9017184333440, 306045200463519,11220008681600256, 441866073895351128,18603606156815235584, 833860238440653331505,39643749441387211150336] |