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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 28 |
| 4 | 224 |
| 5 | 2016 |
| 6 | 20160 |
| 7 | 243840 |
| 8 | 2545920 |
| 9 | 30905280 |
| 10 | 388908036 |
| 11 | 3622707200 |
| 12 | 44117911764 |
| 13 | 387754160640 |
| 14 | 5065246517760 |
| 15 | 67333642371072 |
| 16 | 829993839244416 |
| 17 | 9887956143636480 |
| 18 | 150394118343146496 |
| 19 | 1797812804753817600 |
[1,4,28,224,2016,20160,243840,2545920,30905280, 388908036,3622707200,44117911764,387754160640, 5065246517760,67333642371072,829993839244416, 9887956143636480,150394118343146496, 1797812804753817600] |