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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 14 |
| 4 | 56 |
| 5 | 304 |
| 6 | 1904 |
| 7 | 14048 |
| 8 | 112384 |
| 9 | 1051776 |
| 10 | 10662912 |
| 11 | 120920832 |
| 12 | 1451049984 |
| 13 | 19342651392 |
| 14 | 272576268288 |
| 15 | 4175822315520 |
| 16 | 66813157048320 |
| 17 | 1156746459709440 |
| 18 | 20900477925457920 |
| 19 | 403511454289428480 |
| 20 | 8070229085788569600 |
| 21 | 171907712809736601600 |
[1,4,14,56,304,1904,14048,112384,1051776,10662912, 120920832,1451049984,19342651392,272576268288, 4175822315520,66813157048320,1156746459709440, 20900477925457920,403511454289428480, 8070229085788569600,171907712809736601600] |