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| n | a(n) |
|---|---|
| 0 | 4 |
| 1 | 15 |
| 2 | 85 |
| 3 | 585 |
| 4 | 4369 |
| 5 | 33825 |
| 6 | 266305 |
| 7 | 2113665 |
| 8 | 16843009 |
| 9 | 134480385 |
| 10 | 1074791425 |
| 11 | 8594130945 |
| 12 | 68736258049 |
| 13 | 549822930945 |
| 14 | 4398314962945 |
| 15 | 35185445863425 |
| 16 | 281479271743489 |
| 17 | 2251816993685505 |
| 18 | 18014467229220865 |
[4,15,85,585,4369,33825,266305,2113665,16843009, 134480385,1074791425,8594130945,68736258049, 549822930945,4398314962945,35185445863425, 281479271743489,2251816993685505, 18014467229220865] |