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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 0 |
| 3 | 2 |
| 4 | 8 |
| 5 | 40 |
| 6 | 200 |
| 7 | 1400 |
| 8 | 11200 |
| 9 | 103040 |
| 10 | 1030400 |
| 11 | 11334400 |
| 12 | 135766400 |
| 13 | 1764963200 |
| 14 | 24709484800 |
| 15 | 370687116800 |
| 16 | 5930993868800 |
| 17 | 100826895769600 |
| 18 | 1814871926067200 |
| 19 | 34482566595276800 |
| 20 | 689651331905536000 |
| 21 | 14482682605174784000 |
| 22 | 318619017313845248000 |
[0,0,0,2,8,40,200,1400,11200,103040,1030400, 11334400,135766400,1764963200,24709484800, 370687116800,5930993868800,100826895769600, 1814871926067200,34482566595276800, 689651331905536000,14482682605174784000, 318619017313845248000] |