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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 0 |
| 3 | 6 |
| 4 | 36 |
| 5 | 210 |
| 6 | 1710 |
| 7 | 17304 |
| 8 | 194712 |
| 9 | 2402184 |
| 10 | 32536080 |
| 11 | 481094856 |
| 12 | 7703580456 |
| 13 | 132658888752 |
| 14 | 2443228469136 |
| 15 | 47904722262144 |
| 16 | 995970495769920 |
| 17 | 21879712141853760 |
| 18 | 506301721998264000 |
| 19 | 12306713585213260800 |
| 20 | 313441368701926135680 |
| 21 | 8345931596469584686080 |
[1,0,0,6,36,210,1710,17304,194712,2402184, 32536080,481094856,7703580456,132658888752, 2443228469136,47904722262144,995970495769920, 21879712141853760,506301721998264000, 12306713585213260800,313441368701926135680, 8345931596469584686080] |