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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 12 |
| 4 | 48 |
| 5 | 180 |
| 6 | 528 |
| 7 | 1392 |
| 8 | 3600 |
| 9 | 7920 |
| 10 | 13680 |
| 11 | 21840 |
| 12 | 31872 |
| 13 | 25008 |
[1,4,12,48,180,528,1392,3600,7920,13680,21840, 31872,25008] |