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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 4 |
| 4 | 20 |
| 5 | 60 |
| 6 | 420 |
| 7 | 3360 |
| 8 | 30240 |
| 9 | 151200 |
| 10 | 1663200 |
| 11 | 9979200 |
| 12 | 129729600 |
| 13 | 1816214400 |
| 14 | 27243216000 |
| 15 | 217945728000 |
| 16 | 3705077376000 |
| 17 | 66691392768000 |
| 18 | 633568231296000 |
| 19 | 12671364625920000 |
| 20 | 266098657144320000 |
| 21 | 5854170457175040000 |
| 22 | 134645920515025920000 |
| 23 | 1615751046180311040000 |
[1,1,1,4,20,60,420,3360,30240,151200,1663200, 9979200,129729600,1816214400,27243216000, 217945728000,3705077376000,66691392768000, 633568231296000,12671364625920000, 266098657144320000,5854170457175040000, 134645920515025920000,1615751046180311040000] |