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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 12 |
| 3 | 46 |
| 4 | 220 |
| 5 | 1268 |
| 6 | 8568 |
| 7 | 66456 |
| 8 | 582048 |
| 9 | 5681952 |
| 10 | 61174080 |
| 11 | 720089280 |
| 12 | 9199906560 |
| 13 | 126783809280 |
| 14 | 1874605662720 |
| 15 | 29601115891200 |
| 16 | 497155992883200 |
| 17 | 8849184886886400 |
| 18 | 166399076525875200 |
| 19 | 3296032301811916800 |
| 20 | 68596838245232640000 |
| 21 | 1496490349337948160000 |
[1,4,12,46,220,1268,8568,66456,582048,5681952, 61174080,720089280,9199906560,126783809280, 1874605662720,29601115891200,497155992883200, 8849184886886400,166399076525875200, 3296032301811916800,68596838245232640000, 1496490349337948160000] |