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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 16 |
| 4 | 80 |
| 5 | 456 |
| 6 | 3072 |
| 7 | 23536 |
| 8 | 202304 |
| 9 | 1937920 |
| 10 | 20470400 |
| 11 | 236172288 |
| 12 | 2955465216 |
| 13 | 39893618688 |
| 14 | 577937479680 |
| 15 | 8944476580864 |
| 16 | 147277509541888 |
| 17 | 2570740210700288 |
| 18 | 47418288632692736 |
| 19 | 921669646969167872 |
| 20 | 18829500772271472640 |
| 21 | 403390045252173381632 |
[0,1,4,16,80,456,3072,23536,202304,1937920, 20470400,236172288,2955465216,39893618688, 577937479680,8944476580864,147277509541888, 2570740210700288,47418288632692736, 921669646969167872,18829500772271472640, 403390045252173381632] |