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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 4 |
| 4 | 16 |
| 5 | 80 |
| 6 | 520 |
| 7 | 3640 |
| 8 | 29120 |
| 9 | 259840 |
| 10 | 2598400 |
| 11 | 28582400 |
| 12 | 343235200 |
| 13 | 4462057600 |
| 14 | 62468806400 |
| 15 | 936987251200 |
| 16 | 14991796019200 |
| 17 | 254860532326400 |
| 18 | 4587501779660800 |
| 19 | 87162533813555200 |
| 20 | 1743250676271104000 |
| 21 | 36608259566534656000 |
| 22 | 805381710463762432000 |
[1,1,2,4,16,80,520,3640,29120,259840,2598400, 28582400,343235200,4462057600,62468806400, 936987251200,14991796019200,254860532326400, 4587501779660800,87162533813555200, 1743250676271104000,36608259566534656000, 805381710463762432000] |