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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 40 |
| 4 | 760 |
| 5 | 23200 |
| 6 | 1038400 |
| 7 | 64081600 |
| 8 | 5214880000 |
| 9 | 541085248000 |
| 10 | 69718686400000 |
| 11 | 10921720817920000 |
| 12 | 2044231370959360000 |
| 13 | 450550323286412800000 |
| 14 | 115495483535461427200000 |
| 15 | 34070943029324134912000000 |
| 16 | 11460293146666575236608000000 |
| 17 | 4360020024970859812710400000000 |
| 18 | 1862768688935303816870072320000000 |
[1,1,4,40,760,23200,1038400,64081600,5214880000, 541085248000,69718686400000,10921720817920000, 2044231370959360000,450550323286412800000, 115495483535461427200000, 34070943029324134912000000, 11460293146666575236608000000, 4360020024970859812710400000000, 1862768688935303816870072320000000] |