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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 6 |
| 3 | 18 |
| 4 | 110 |
| 5 | 498 |
| 6 | 3366 |
| 7 | 17282 |
| 8 | 122958 |
| 9 | 672930 |
| 10 | 4938758 |
| 11 | 28103730 |
| 12 | 210595182 |
| 13 | 1230391058 |
| 14 | 9358456230 |
| 15 | 55727128866 |
| 16 | 428643977422 |
| 17 | 2589488117826 |
| 18 | 20092671283974 |
| 19 | 122759098980690 |
| 20 | 959216278565742 |
| 21 | 5913900861617970 |
[1,1,6,18,110,498,3366,17282,122958,672930, 4938758,28103730,210595182,1230391058,9358456230, 55727128866,428643977422,2589488117826, 20092671283974,122759098980690,959216278565742, 5913900861617970] |