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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 3 |
| 3 | 12 |
| 4 | 73 |
| 5 | 556 |
| 6 | 4737 |
| 7 | 44122 |
| 8 | 453441 |
| 9 | 5226984 |
| 10 | 69703201 |
| 11 | 1132247854 |
| 12 | 22865062497 |
| 13 | 541274154604 |
| 14 | 13810143608913 |
| 15 | 361165919643666 |
| 16 | 9534875308290433 |
| 17 | 254667431232941008 |
| 18 | 6928956222876625473 |
| 19 | 193066959857334885334 |
| 20 | 5521621116955849623201 |
[1,1,3,12,73,556,4737,44122,453441,5226984, 69703201,1132247854,22865062497,541274154604, 13810143608913,361165919643666,9534875308290433, 254667431232941008,6928956222876625473, 193066959857334885334,5521621116955849623201] |