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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 4 |
| 2 | 33 |
| 3 | 232 |
| 4 | 1596 |
| 5 | 10945 |
| 6 | 75024 |
| 7 | 514228 |
| 8 | 3524577 |
| 9 | 24157816 |
| 10 | 165580140 |
| 11 | 1134903169 |
| 12 | 7778742048 |
| 13 | 53316291172 |
| 14 | 365435296161 |
| 15 | 2504730781960 |
| 16 | 17167680177564 |
| 17 | 117669030460993 |
| 18 | 806515533049392 |
| 19 | 5527939700884756 |
| 20 | 37889062373143905 |
| 21 | 259695496911122584 |
[0,4,33,232,1596,10945,75024,514228,3524577, 24157816,165580140,1134903169,7778742048, 53316291172,365435296161,2504730781960, 17167680177564,117669030460993,806515533049392, 5527939700884756,37889062373143905, 259695496911122584] |