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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 20 |
| 4 | 65 |
| 5 | 226 |
| 6 | 883 |
| 7 | 3947 |
| 8 | 20089 |
| 9 | 115036 |
| 10 | 732171 |
| 11 | 5126901 |
| 12 | 39165917 |
| 13 | 324138010 |
| 14 | 2888934623 |
| 15 | 27587288507 |
| 16 | 281001801969 |
| 17 | 3041152133848 |
| 18 | 34849036364659 |
| 19 | 421526126267265 |
| 20 | 5367037330561365 |
| 21 | 71752003756908550 |
[1,2,6,20,65,226,883,3947,20089,115036,732171, 5126901,39165917,324138010,2888934623,27587288507, 281001801969,3041152133848,34849036364659, 421526126267265,5367037330561365, 71752003756908550] |