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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 23 |
| 5 | 104 |
| 6 | 531 |
| 7 | 2977 |
| 8 | 17980 |
| 9 | 115408 |
| 10 | 779418 |
| 11 | 5496355 |
| 12 | 40229830 |
| 13 | 304165473 |
| 14 | 2366281302 |
| 15 | 18880731580 |
[1,1,2,6,23,104,531,2977,17980,115408,779418, 5496355,40229830,304165473,2366281302,18880731580] |