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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 3 |
| 2 | 16 |
| 3 | 75 |
| 4 | 356 |
| 5 | 1770 |
| 6 | 9306 |
| 7 | 51821 |
| 8 | 305232 |
| 9 | 1897227 |
| 10 | 12411650 |
| 11 | 85234864 |
| 12 | 612903324 |
| 13 | 4604004691 |
| 14 | 36050248752 |
| 15 | 293659886235 |
| 16 | 2483993559376 |
| 17 | 21781908622738 |
| 18 | 197701336871190 |
| 19 | 1854695911048053 |
| 20 | 17960309481562600 |
| 21 | 179309794875495795 |
[0,3,16,75,356,1770,9306,51821,305232,1897227, 12411650,85234864,612903324,4604004691, 36050248752,293659886235,2483993559376, 21781908622738,197701336871190,1854695911048053, 17960309481562600,179309794875495795] |