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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 2 |
| 3 | 6 |
| 4 | 36 |
| 5 | 240 |
| 6 | 1920 |
| 7 | 17640 |
| 8 | 183120 |
| 9 | 2116800 |
| 10 | 26943840 |
| 11 | 374220000 |
| 12 | 5628934080 |
| 13 | 91122071040 |
| 14 | 1579034096640 |
| 15 | 29155689763200 |
| 16 | 571308920582400 |
| 17 | 11838533804697600 |
| 18 | 258608278645516800 |
| 19 | 5938673374272038400 |
| 20 | 143003892952893772800 |
| 21 | 3602735624977961472000 |
[1,0,2,6,36,240,1920,17640,183120,2116800, 26943840,374220000,5628934080,91122071040, 1579034096640,29155689763200,571308920582400, 11838533804697600,258608278645516800, 5938673374272038400,143003892952893772800, 3602735624977961472000] |