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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 20 |
| 5 | 72 |
| 6 | 274 |
| 7 | 1088 |
| 8 | 4470 |
| 9 | 18884 |
| 10 | 81652 |
| 11 | 360054 |
| 12 | 1614618 |
| 13 | 7346688 |
| 14 | 33856008 |
| 15 | 157777908 |
| 16 | 742637416 |
[1,1,2,6,20,72,274,1088,4470,18884,81652,360054, 1614618,7346688,33856008,157777908,742637416] |