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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 15 |
| 4 | 120 |
| 5 | 780 |
| 6 | 9744 |
| 7 | 92085 |
| 8 | 1589760 |
| 9 | 19983915 |
| 10 | 445344000 |
| 11 | 7053735150 |
| 12 | 194162549760 |
| 13 | 3735730993995 |
| 14 | 123152970608640 |
| 15 | 2803291544678625 |
| 16 | 108175188829962240 |
| 17 | 2855475766145702700 |
| 18 | 126726160086516695040 |
| 19 | 3818865908630429932275 |
| 20 | 192201344414226343526400 |
[1,1,4,15,120,780,9744,92085,1589760,19983915, 445344000,7053735150,194162549760,3735730993995, 123152970608640,2803291544678625, 108175188829962240,2855475766145702700, 126726160086516695040,3818865908630429932275, 192201344414226343526400] |