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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 204 |
| 5 | 1260 |
| 6 | 37440 |
| 7 | 299880 |
| 8 | 11002320 |
| 9 | 204860880 |
| 10 | 6618628800 |
| 11 | 92924647200 |
| 12 | 8181137764800 |
| 13 | 124123075876800 |
| 14 | 7211104918617600 |
| 15 | 288085376346768000 |
| 16 | 14964000305173920000 |
| 17 | 340302035937191328000 |
| 18 | 42619767305209750656000 |
[1,1,4,18,204,1260,37440,299880,11002320, 204860880,6618628800,92924647200,8181137764800, 124123075876800,7211104918617600, 288085376346768000,14964000305173920000, 340302035937191328000,42619767305209750656000] |