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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 6 |
| 5 | 25 |
| 6 | 115 |
| 7 | 649 |
| 8 | 4046 |
| 9 | 29674 |
| 10 | 228030 |
| 11 | 1987700 |
| 12 | 18402704 |
| 13 | 188255116 |
| 14 | 2030067605 |
| 15 | 23829298479 |
| 16 | 293949166112 |
| 17 | 3909410101509 |
[1,1,1,2,6,25,115,649,4046,29674,228030,1987700, 18402704,188255116,2030067605,23829298479, 293949166112,3909410101509] |