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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 2 |
| 3 | 4 |
| 4 | 15 |
| 5 | 31 |
| 6 | 126 |
| 7 | 927 |
| 8 | 3933 |
| 9 | 31429 |
| 10 | 472959 |
| 11 | 4005742 |
| 12 | 63878219 |
| 13 | 1917687587 |
| 14 | 32433255622 |
| 15 | 1032580394091 |
| 16 | 61890749905798 |
| 17 | 2090664356692297 |
| 18 | 132950076813594995 |
| 19 | 15915454829832076325 |
| 20 | 1073793991171675728498 |
[1,2,2,4,15,31,126,927,3933,31429,472959,4005742, 63878219,1917687587,32433255622,1032580394091, 61890749905798,2090664356692297, 132950076813594995,15915454829832076325, 1073793991171675728498] |