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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 3 |
| 5 | 6 |
| 6 | 15 |
| 7 | 90 |
| 8 | 310 |
| 9 | 1860 |
| 10 | 8280 |
| 11 | 163560 |
| 12 | 1346940 |
| 13 | 21476700 |
[1,1,2,3,6,15,90,310,1860,8280,163560,1346940, 21476700] |