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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 0 |
| 3 | 6 |
| 4 | 12 |
| 5 | 40 |
| 6 | 1620 |
| 7 | 13608 |
| 8 | 117600 |
| 9 | 2924640 |
| 10 | 49603680 |
| 11 | 782147520 |
| 12 | 19083936960 |
| 13 | 463369645440 |
| 14 | 10836652514688 |
| 15 | 304533583200000 |
| 16 | 9218842256332800 |
| 17 | 281872333420554240 |
| 18 | 9421579421176089600 |
| 19 | 338543319734116116480 |
| 20 | 12590519274541116518400 |
[1,0,0,6,12,40,1620,13608,117600,2924640,49603680, 782147520,19083936960,463369645440,10836652514688, 304533583200000,9218842256332800, 281872333420554240,9421579421176089600, 338543319734116116480,12590519274541116518400] |