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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 4 |
| 4 | 13 |
| 5 | 20 |
| 6 | 125 |
| 7 | 403 |
| 8 | 1438 |
| 9 | 4718 |
| 10 | 17700 |
| 11 | 65811 |
| 12 | 266345 |
| 13 | 1036625 |
| 14 | 3817494 |
| 15 | 15399048 |
[1,1,2,4,13,20,125,403,1438,4718,17700,65811, 266345,1036625,3817494,15399048] |