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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 120 |
| 4 | 800 |
| 5 | 5600 |
| 6 | 40000 |
| 7 | 288000 |
| 8 | 2080000 |
| 9 | 15040000 |
| 10 | 108800000 |
| 11 | 787200000 |
| 12 | 5696000000 |
| 13 | 41216000000 |
| 14 | 298240000000 |
| 15 | 2158080000000 |
| 16 | 15616000000000 |
| 17 | 112998400000000 |
| 18 | 817664000000000 |
| 19 | 5916672000000000 |
| 20 | 42813440000000000 |
[1,4,20,120,800,5600,40000,288000,2080000, 15040000,108800000,787200000,5696000000, 41216000000,298240000000,2158080000000, 15616000000000,112998400000000,817664000000000, 5916672000000000,42813440000000000] |