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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 3960 |
| 3 | 168960 |
| 4 | 2094840 |
| 5 | 16625664 |
| 6 | 80652000 |
| 7 | 343664640 |
| 8 | 1073639160 |
| 9 | 3299450880 |
| 10 | 8000389584 |
| 11 | 20081064960 |
| 12 | 41278084320 |
| 13 | 90307768320 |
| 14 | 165298921920 |
| 15 | 327406565376 |
| 16 | 549720551160 |
| 17 | 1009911767040 |
| 18 | 1587054299160 |
| 19 | 2748047385600 |
| 20 | 4095742639824 |
| 21 | 6764533923840 |
[1,0,3960,168960,2094840,16625664,80652000, 343664640,1073639160,3299450880,8000389584, 20081064960,41278084320,90307768320,165298921920, 327406565376,549720551160,1009911767040, 1587054299160,2748047385600,4095742639824, 6764533923840] |