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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 2 |
| 4 | 9 |
| 5 | 24 |
| 6 | 130 |
| 7 | 720 |
| 8 | 8505 |
| 9 | 35840 |
| 10 | 412776 |
| 11 | 3628800 |
| 12 | 42030450 |
| 13 | 479001600 |
| 14 | 7019298000 |
| 15 | 82614884352 |
| 16 | 1886805545625 |
| 17 | 20922789888000 |
| 18 | 374426276224000 |
| 19 | 6402373705728000 |
| 20 | 134987215801622184 |
| 21 | 2379913632645120000 |
[0,1,2,9,24,130,720,8505,35840,412776,3628800, 42030450,479001600,7019298000,82614884352, 1886805545625,20922789888000,374426276224000, 6402373705728000,134987215801622184, 2379913632645120000] |