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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 12 |
| 4 | 48 |
| 5 | 240 |
| 6 | 1800 |
| 7 | 12600 |
| 8 | 100800 |
| 9 | 967680 |
| 10 | 9676800 |
| 11 | 106444800 |
| 12 | 1297296000 |
| 13 | 16864848000 |
| 14 | 236107872000 |
| 15 | 3552515366400 |
| 16 | 56840245862400 |
| 17 | 966284179660800 |
| 18 | 17402007419596800 |
| 19 | 330638140972339200 |
| 20 | 6612762819446784000 |
| 21 | 138878156300083200000 |
[1,1,2,12,48,240,1800,12600,100800,967680,9676800, 106444800,1297296000,16864848000,236107872000, 3552515366400,56840245862400,966284179660800, 17402007419596800,330638140972339200, 6612762819446784000,138878156300083200000] |