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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 0 |
| 3 | 2 |
| 4 | 15 |
| 5 | 123 |
| 6 | 1118 |
| 7 | 11344 |
| 8 | 127831 |
| 9 | 1590245 |
| 10 | 21700716 |
| 11 | 322880256 |
| 12 | 5209007463 |
| 13 | 90661989607 |
| 14 | 1694616510154 |
| 15 | 33876697720832 |
| 16 | 721588072472639 |
| 17 | 16321494271570569 |
| 18 | 390811944752490542 |
| 19 | 9878354899591168000 |
| 20 | 262896868506265373394 |
| 21 | 7349159002086450661211 |
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