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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 232 |
| 4 | 2156 |
| 5 | 21944 |
| 6 | 240280 |
| 7 | 2787320 |
| 8 | 33820044 |
| 9 | 424925872 |
| 10 | 5486681368 |
| 11 | 72398776344 |
| 12 | 972270849512 |
| 13 | 13247921422480 |
| 14 | 182729003683352 |
| 15 | 2546778437385032 |
| 16 | 35816909974343308 |
| 17 | 507700854900783784 |
| 18 | 7246857513425470288 |
| 19 | 104083322583897779656 |
[1,4,28,232,2156,21944,240280,2787320,33820044, 424925872,5486681368,72398776344,972270849512, 13247921422480,182729003683352,2546778437385032, 35816909974343308,507700854900783784, 7246857513425470288,104083322583897779656] |