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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 2 |
| 3 | 8 |
| 4 | 66 |
| 5 | 400 |
| 6 | 2722 |
| 7 | 17688 |
| 8 | 117026 |
| 9 | 768800 |
| 10 | 5064642 |
| 11 | 33328168 |
| 12 | 219411586 |
| 13 | 1444225200 |
| 14 | 9506897762 |
| 15 | 62579419448 |
| 16 | 411934939746 |
| 17 | 2711589889600 |
| 18 | 17849253534082 |
| 19 | 117494042259528 |
| 20 | 773413479117506 |
| 21 | 5091052634882000 |
[0,2,8,66,400,2722,17688,117026,768800,5064642, 33328168,219411586,1444225200,9506897762, 62579419448,411934939746,2711589889600, 17849253534082,117494042259528,773413479117506, 5091052634882000] |