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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 220 |
| 4 | 60092152 |
| 5 | 203720181459953921762400 |
| 6 | 7088043372247785801830314829178419617696182324188730917543544992 |
[1,1,4,220,60092152,203720181459953921762400, 70880433722477858018303148291784196176961823241887\ 30917543544992] |