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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 4 |
| 5 | 10 |
| 6 | 40 |
| 7 | 210 |
| 8 | 5264 |
| 9 | 45360 |
| 10 | 409800 |
| 11 | 4065600 |
| 12 | 77948640 |
| 13 | 1183422240 |
| 14 | 17527233360 |
| 15 | 267109642800 |
| 16 | 5422495921920 |
| 17 | 110998923235200 |
| 18 | 2270809072896000 |
| 19 | 47142009514454400 |
| 20 | 1116394268619772800 |
| 21 | 27963045712157472000 |
| 22 | 718066383283082803200 |
[1,0,0,0,4,10,40,210,5264,45360,409800,4065600, 77948640,1183422240,17527233360,267109642800, 5422495921920,110998923235200,2270809072896000, 47142009514454400,1116394268619772800, 27963045712157472000,718066383283082803200] |