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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 120 |
| 4 | 74455920 |
| 5 | 479301523749226879680 |
| 6 | 1336367227479573478314645756081634359006026455038720 |
[1,4,120,74455920,479301523749226879680, 13363672274795734783146457560816343590060264550387\ 20] |