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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 0 |
| 5 | 120 |
| 6 | 1800 |
| 7 | 21000 |
| 8 | 235200 |
| 9 | 2693880 |
| 10 | 34133400 |
| 11 | 509823600 |
| 12 | 9012207600 |
| 13 | 180053908320 |
| 14 | 3870208261920 |
| 15 | 87083930169600 |
| 16 | 2034907999488000 |
| 17 | 49491370609706880 |
| 18 | 1259740748821328640 |
| 19 | 33710658096392887680 |
| 20 | 949893399326820528000 |
[1,0,0,0,0,120,1800,21000,235200,2693880,34133400, 509823600,9012207600,180053908320,3870208261920, 87083930169600,2034907999488000,49491370609706880, 1259740748821328640,33710658096392887680, 949893399326820528000] |