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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 42 |
| 3 | 660 |
| 4 | 12810 |
| 5 | 281736 |
| 6 | 6727644 |
| 7 | 170316432 |
| 8 | 4504487130 |
| 9 | 123255492360 |
| 10 | 3465702008340 |
| 11 | 99645553785960 |
| 12 | 2918768920720380 |
| 13 | 86852063374902000 |
| 14 | 2619552500788984200 |
| 15 | 79939673971478231760 |
[1,4,42,660,12810,281736,6727644,170316432, 4504487130,123255492360,3465702008340, 99645553785960,2918768920720380,86852063374902000, 2619552500788984200,79939673971478231760] |