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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 40 |
| 4 | 330 |
| 5 | 3048 |
| 6 | 30156 |
| 7 | 312528 |
| 8 | 3349170 |
| 9 | 36809960 |
| 10 | 412651668 |
| 11 | 4700098416 |
| 12 | 54237852708 |
| 13 | 632762593680 |
| 14 | 7450815536280 |
| 15 | 88435205367456 |
| 16 | 1056940049423682 |
| 17 | 12708927083800296 |
| 18 | 153636691533864900 |
| 19 | 1866178021496170800 |
[1,2,6,40,330,3048,30156,312528,3349170,36809960, 412651668,4700098416,54237852708,632762593680, 7450815536280,88435205367456,1056940049423682, 12708927083800296,153636691533864900, 1866178021496170800] |