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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 20 |
| 5 | 157 |
| 6 | 902 |
| 7 | 6895 |
| 8 | 77352 |
| 9 | 837145 |
| 10 | 9239882 |
| 11 | 124480059 |
| 12 | 1864519868 |
| 13 | 28461793141 |
| 14 | 470175087566 |
| 15 | 8596601614567 |
| 16 | 165789417707088 |
| 17 | 3347308657310641 |
| 18 | 72262366126097810 |
| 19 | 1655981959056540147 |
| 20 | 39676337445225379172 |
| 21 | 996780437366905850317 |
[0,1,2,3,20,157,902,6895,77352,837145,9239882, 124480059,1864519868,28461793141,470175087566, 8596601614567,165789417707088,3347308657310641, 72262366126097810,1655981959056540147, 39676337445225379172,996780437366905850317] |