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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 0 |
| 3 | 2 |
| 4 | 6 |
| 5 | 30 |
| 6 | 180 |
| 7 | 1260 |
| 8 | 10080 |
| 9 | 90720 |
| 10 | 887040 |
| 11 | 9432360 |
| 12 | 107012160 |
| 13 | 1300448160 |
| 14 | 16716051384 |
| 15 | 228160919040 |
| 16 | 3276565988040 |
| 17 | 49664631128706 |
| 18 | 788840324793320 |
[1,0,2,6,30,180,1260,10080,90720,887040,9432360, 107012160,1300448160,16716051384,228160919040, 3276565988040,49664631128706,788840324793320] |