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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 18 |
| 5 | 30 |
| 6 | 270 |
| 7 | 606 |
| 8 | 123120 |
| 9 | 888456 |
| 10 | 23070450 |
| 11 | 238550160 |
| 12 | 8282903640 |
| 13 | 72789145650 |
| 14 | 15681266370000 |
| 15 | 18216437241240 |
[1,2,6,18,30,270,606,123120,888456,23070450, 238550160,8282903640,72789145650,15681266370000, 18216437241240] |