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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 3 |
| 4 | 9 |
| 5 | 54 |
| 6 | 285 |
| 7 | 2160 |
| 8 | 15825 |
| 9 | 151200 |
| 10 | 1411095 |
| 11 | 16329600 |
| 12 | 185067855 |
| 13 | 2514758400 |
| 14 | 33530101605 |
| 15 | 523069747200 |
| 16 | 8020402655265 |
| 17 | 141228831744000 |
| 18 | 2447966414868975 |
| 19 | 48017802792960000 |
| 20 | 928344187296100575 |
| 21 | 20071441567457280000 |
| 22 | 428190753438433910925 |
[1,1,1,3,9,54,285,2160,15825,151200,1411095, 16329600,185067855,2514758400,33530101605, 523069747200,8020402655265,141228831744000, 2447966414868975,48017802792960000, 928344187296100575,20071441567457280000, 428190753438433910925] |