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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 6 |
| 4 | 18 |
| 5 | 60 |
| 6 | 240 |
| 7 | 1008 |
| 8 | 4908 |
| 9 | 28080 |
| 10 | 165840 |
| 11 | 940896 |
| 12 | 5174688 |
| 13 | 28304640 |
| 14 | 155696585 |
| 15 | 865095552 |
| 16 | 4861712448 |
| 17 | 27554100480 |
| 18 | 156752183808 |
| 19 | 892662156288 |
| 20 | 5087657135000 |
| 21 | 29043991855542 |
| 22 | 166186204664832 |
| 23 | 953309368018944 |
| 24 | 5481528861090816 |
| 25 | 31581437286776820 |
[0,1,6,18,60,240,1008,4908,28080,165840,940896, 5174688,28304640,155696585,865095552,4861712448, 27554100480,156752183808,892662156288, 5087657135000,29043991855542,166186204664832, 953309368018944,5481528861090816, 31581437286776820] |