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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 6 |
| 2 | 40 |
| 3 | 280 |
| 4 | 2000 |
| 5 | 14400 |
| 6 | 104000 |
| 7 | 752000 |
| 8 | 5440000 |
| 9 | 39360000 |
| 10 | 284800000 |
| 11 | 2060800000 |
| 12 | 14912000000 |
| 13 | 107904000000 |
| 14 | 780800000000 |
| 15 | 5649920000000 |
| 16 | 40883200000000 |
| 17 | 295833600000000 |
| 18 | 2140672000000000 |
| 19 | 15490048000000000 |
[1,6,40,280,2000,14400,104000,752000,5440000, 39360000,284800000,2060800000,14912000000, 107904000000,780800000000,5649920000000, 40883200000000,295833600000000,2140672000000000, 15490048000000000] |