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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 6 |
| 5 | 20 |
| 6 | 105 |
| 7 | 724 |
| 8 | 7805 |
| 9 | 126777 |
| 10 | 3122221 |
| 11 | 112392709 |
| 12 | 5736233644 |
| 13 | 404604893810 |
[1,1,2,6,20,105,724,7805,126777,3122221,112392709, 5736233644,404604893810] |