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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 6 |
| 5 | 21 |
| 6 | 91 |
| 7 | 462 |
| 8 | 2646 |
| 9 | 16841 |
| 10 | 117931 |
| 11 | 900922 |
| 12 | 7456086 |
| 13 | 66453661 |
| 14 | 634585771 |
| 15 | 6463837382 |
| 16 | 69955114326 |
| 17 | 801626558481 |
| 18 | 9696295037611 |
| 19 | 123457062745842 |
| 20 | 1650499351041366 |
| 21 | 23116291464379301 |
| 22 | 338474628835617451 |
[1,1,1,2,6,21,91,462,2646,16841,117931,900922, 7456086,66453661,634585771,6463837382,69955114326, 801626558481,9696295037611,123457062745842, 1650499351041366,23116291464379301, 338474628835617451] |