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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 40 |
| 3 | 3280 |
| 4 | 21523360 |
| 5 | 926510094425920 |
| 6 | 1716841910146256242328924544640 |
| 7 | 5895092288869291585760436430706259332839105796137920554548480 |
[1,4,40,3280,21523360,926510094425920, 1716841910146256242328924544640, 58950922888692915857604364307062593328391057961379\ 20554548480] |