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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 0 |
| 3 | 3 |
| 4 | 18 |
| 5 | 100 |
| 6 | 615 |
| 7 | 4326 |
| 8 | 34636 |
| 9 | 311760 |
| 10 | 3117645 |
| 11 | 34294150 |
| 12 | 411529866 |
| 13 | 5349888336 |
| 14 | 74898436795 |
| 15 | 1123476552030 |
| 16 | 17975624832600 |
| 17 | 305585622154336 |
| 18 | 5500541198778201 |
| 19 | 104510282776785990 |
| 20 | 2090205655535719990 |
| 21 | 43894318766250120000 |
[0,0,0,3,18,100,615,4326,34636,311760,3117645, 34294150,411529866,5349888336,74898436795, 1123476552030,17975624832600,305585622154336, 5500541198778201,104510282776785990, 2090205655535719990,43894318766250120000] |