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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 4 |
| 3 | 6 |
| 4 | 136 |
| 5 | 900 |
| 6 | 16308 |
| 7 | 229320 |
| 8 | 4691104 |
| 9 | 99156960 |
| 10 | 2481162480 |
| 11 | 67862678400 |
| 12 | 2063842827264 |
| 13 | 68473763804160 |
| 14 | 2468786906210688 |
| 15 | 96048626176339200 |
| 16 | 4010912604492410880 |
| 17 | 178968539487145282560 |
| 18 | 8496991445958129576960 |
| 19 | 427734144995749047152640 |
[1,0,4,6,136,900,16308,229320,4691104,99156960, 2481162480,67862678400,2063842827264, 68473763804160,2468786906210688,96048626176339200, 4010912604492410880,178968539487145282560, 8496991445958129576960,427734144995749047152640] |