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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 9 |
| 4 | 53 |
| 5 | 464 |
| 6 | 4687 |
| 7 | 61479 |
| 8 | 886258 |
| 9 | 15524575 |
| 10 | 291505295 |
| 11 | 6394185264 |
| 12 | 148118036273 |
| 13 | 3904856867873 |
| 14 | 107649269291290 |
| 15 | 3315317751482457 |
| 16 | 106031435475881053 |
| 17 | 3736096118740095232 |
| 18 | 136008862573108676543 |
| 19 | 5396412601537476189351 |
| 20 | 220341893750220550211546 |
| 21 | 9721518437451024623900879 |
[1,1,2,9,53,464,4687,61479,886258,15524575, 291505295,6394185264,148118036273,3904856867873, 107649269291290,3315317751482457, 106031435475881053,3736096118740095232, 136008862573108676543,5396412601537476189351, 220341893750220550211546, 9721518437451024623900879] |