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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 39 |
| 4 | 500 |
| 5 | 7990 |
| 6 | 156684 |
| 7 | 3640392 |
| 8 | 97543088 |
| 9 | 2960758800 |
| 10 | 100428661440 |
| 11 | 3764849536800 |
| 12 | 154567280328768 |
| 13 | 6897265807262064 |
| 14 | 332386213584653760 |
| 15 | 17204016957686536320 |
| 16 | 951852354201532742400 |
| 17 | 56059949872552858763520 |
| 18 | 3501729575599545174352896 |
| 19 | 231227806715994322631352960 |
[1,1,4,39,500,7990,156684,3640392,97543088, 2960758800,100428661440,3764849536800, 154567280328768,6897265807262064, 332386213584653760,17204016957686536320, 951852354201532742400,56059949872552858763520, 3501729575599545174352896, 231227806715994322631352960] |