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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 28 |
| 3 | 132 |
| 4 | 671 |
| 5 | 3444 |
| 6 | 17558 |
| 7 | 89613 |
| 8 | 457188 |
| 9 | 2333009 |
| 10 | 11905494 |
| 11 | 60751194 |
| 12 | 310004765 |
| 13 | 1581914211 |
| 14 | 8072288131 |
| 15 | 41191772847 |
| 16 | 210195946958 |
| 17 | 1072600980393 |
| 18 | 5473335132061 |
| 19 | 27929675517731 |
| 20 | 142521288635027 |
| 21 | 727266512789731 |
[4,28,132,671,3444,17558,89613,457188,2333009, 11905494,60751194,310004765,1581914211,8072288131, 41191772847,210195946958,1072600980393, 5473335132061,27929675517731,142521288635027, 727266512789731] |