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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 21 |
| 2 | 420 |
| 3 | 8400 |
| 4 | 168000 |
| 5 | 3360000 |
| 6 | 67200000 |
| 7 | 1344000000 |
| 8 | 26880000000 |
| 9 | 537600000000 |
| 10 | 10752000000000 |
| 11 | 215039999999790 |
| 12 | 4300799999991600 |
| 13 | 86015999999748210 |
| 14 | 1720319999993288400 |
| 15 | 34406399999832252000 |
[1,21,420,8400,168000,3360000,67200000,1344000000, 26880000000,537600000000,10752000000000, 215039999999790,4300799999991600, 86015999999748210,1720319999993288400, 34406399999832252000] |