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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 23 |
| 3 | 155 |
| 4 | 1143 |
| 5 | 8932 |
| 6 | 72682 |
| 7 | 609348 |
| 8 | 5227035 |
| 9 | 45659020 |
| 10 | 404756300 |
| 11 | 3632075109 |
| 12 | 32928392154 |
| 13 | 301152242600 |
| 14 | 2775117150576 |
| 15 | 25741623112539 |
| 16 | 240162703635495 |
| 17 | 2252187478291088 |
| 18 | 21217451539791085 |
| 19 | 200709823787548845 |
| 20 | 1905712342347978340 |
[1,4,23,155,1143,8932,72682,609348,5227035, 45659020,404756300,3632075109,32928392154, 301152242600,2775117150576,25741623112539, 240162703635495,2252187478291088, 21217451539791085,200709823787548845, 1905712342347978340] |