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| n | a(n) |
|---|---|
| 2 | 1 |
| 3 | 4 |
| 4 | 22 |
| 5 | 124 |
| 6 | 726 |
| 7 | 4352 |
| 8 | 26614 |
| 9 | 165204 |
| 10 | 1037672 |
| 11 | 6580424 |
| 12 | 42062040 |
| 13 | 270661328 |
| 14 | 1751614248 |
| 15 | 11391756176 |
| 16 | 74406502814 |
| 17 | 487838450116 |
| 18 | 3209229661682 |
| 19 | 21175301453040 |
| 20 | 140097533633112 |
| 21 | 929160187609096 |
[1,4,22,124,726,4352,26614,165204,1037672,6580424, 42062040,270661328,1751614248,11391756176, 74406502814,487838450116,3209229661682, 21175301453040,140097533633112,929160187609096] |