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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 21 |
| 2 | 420 |
| 3 | 8400 |
| 4 | 168000 |
| 5 | 3360000 |
| 6 | 67199790 |
| 7 | 1343991600 |
| 8 | 26879748210 |
| 9 | 537593288400 |
| 10 | 10751832252000 |
| 11 | 215035974720000 |
| 12 | 4300706088043890 |
| 13 | 86013853634593500 |
| 14 | 1720271710182898110 |
| 15 | 34405326953812846500 |
[1,21,420,8400,168000,3360000,67199790,1343991600, 26879748210,537593288400,10751832252000, 215035974720000,4300706088043890, 86013853634593500,1720271710182898110, 34405326953812846500] |