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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 24 |
| 4 | 228 |
| 5 | 2820 |
| 6 | 44400 |
| 7 | 840000 |
| 8 | 18669840 |
| 9 | 475871760 |
| 10 | 13698296640 |
| 11 | 439402803840 |
| 12 | 15545690233920 |
| 13 | 601352177025600 |
| 14 | 25251437978807040 |
| 15 | 1143932660001331200 |
| 16 | 55612090342967558400 |
| 17 | 2887929114414030086400 |
| 18 | 159548423949650274739200 |
[1,1,4,24,228,2820,44400,840000,18669840, 475871760,13698296640,439402803840,15545690233920, 601352177025600,25251437978807040, 1143932660001331200,55612090342967558400, 2887929114414030086400,159548423949650274739200] |