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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 240 |
| 3 | 3640 |
| 4 | 23790 |
| 5 | 99456 |
| 6 | 323680 |
| 7 | 895680 |
| 8 | 2139780 |
| 9 | 4528640 |
| 10 | 8971872 |
| 11 | 16946280 |
| 12 | 29822520 |
| 13 | 49365120 |
| 14 | 80003520 |
| 15 | 127221568 |
| 16 | 193388910 |
| 17 | 282616320 |
| 18 | 409634640 |
| 19 | 590913960 |
| 20 | 825575016 |
| 21 | 1115613440 |
[1,0,240,3640,23790,99456,323680,895680,2139780, 4528640,8971872,16946280,29822520,49365120, 80003520,127221568,193388910,282616320,409634640, 590913960,825575016,1115613440] |