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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 22 |
| 3 | 142 |
| 4 | 1007 |
| 5 | 7590 |
| 6 | 59683 |
| 7 | 484112 |
| 8 | 4021061 |
| 9 | 34029532 |
| 10 | 292373296 |
| 11 | 2543542676 |
| 12 | 22360917140 |
| 13 | 198341377680 |
| 14 | 1772860026933 |
| 15 | 15952960500612 |
| 16 | 144397901220980 |
| 17 | 1313835276189792 |
| 18 | 12009823111155481 |
| 19 | 110240431974732436 |
| 20 | 1015727265740887873 |
[1,4,22,142,1007,7590,59683,484112,4021061, 34029532,292373296,2543542676,22360917140, 198341377680,1772860026933,15952960500612, 144397901220980,1313835276189792, 12009823111155481,110240431974732436, 1015727265740887873] |