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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 252 |
| 3 | 98984058271 |
| 4 | 2556622439429326953984 |
| 5 | 1840612120873072393197860408401920 |
| 6 | 20856759061804812713740241716791439872159072000 |
[0,252,98984058271,2556622439429326953984, 1840612120873072393197860408401920, 20856759061804812713740241716791439872159072000] |