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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 21 |
| 2 | 420 |
| 3 | 8400 |
| 4 | 168000 |
| 5 | 3360000 |
| 6 | 67200000 |
| 7 | 1344000000 |
| 8 | 26879999790 |
| 9 | 537599991600 |
| 10 | 10751999748210 |
| 11 | 215039993288400 |
| 12 | 4300799832252000 |
| 13 | 86015995974720000 |
| 14 | 1720319906088000000 |
| 15 | 34406397853632000000 |
[1,21,420,8400,168000,3360000,67200000,1344000000, 26879999790,537599991600,10751999748210, 215039993288400,4300799832252000, 86015995974720000,1720319906088000000, 34406397853632000000] |