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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 30 |
| 4 | 304 |
| 5 | 3950 |
| 6 | 62736 |
| 7 | 1177288 |
| 8 | 25489024 |
| 9 | 625404060 |
| 10 | 17149867840 |
| 11 | 519779815016 |
| 12 | 17253698140416 |
| 13 | 622521514670536 |
| 14 | 24257636227003648 |
| 15 | 1015256151136695840 |
| 16 | 45421939756667293696 |
| 17 | 2163253599528596482832 |
| 18 | 109270502354027312243712 |
[1,1,4,30,304,3950,62736,1177288,25489024, 625404060,17149867840,519779815016,17253698140416, 622521514670536,24257636227003648, 1015256151136695840,45421939756667293696, 2163253599528596482832,109270502354027312243712] |