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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 3 |
| 2 | 10 |
| 3 | 40 |
| 4 | 200 |
| 5 | 1200 |
| 6 | 8000 |
| 7 | 56000 |
| 8 | 400000 |
| 9 | 2880000 |
| 10 | 20800000 |
| 11 | 150400000 |
| 12 | 1088000000 |
| 13 | 7872000000 |
| 14 | 56960000000 |
| 15 | 412160000000 |
| 16 | 2982400000000 |
| 17 | 21580800000000 |
| 18 | 156160000000000 |
| 19 | 1129984000000000 |
| 20 | 8176640000000000 |
[1,3,10,40,200,1200,8000,56000,400000,2880000, 20800000,150400000,1088000000,7872000000, 56960000000,412160000000,2982400000000, 21580800000000,156160000000000,1129984000000000, 8176640000000000] |