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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 3 |
| 3 | 12 |
| 4 | 63 |
| 5 | 405 |
| 6 | 3075 |
| 7 | 26880 |
| 8 | 265545 |
| 9 | 2922885 |
| 10 | 35447895 |
| 11 | 469396620 |
| 12 | 6736095135 |
| 13 | 104102463465 |
| 14 | 1723322736135 |
| 15 | 30416726340000 |
| 16 | 570089983287825 |
| 17 | 11306156398562025 |
| 18 | 236514323713142475 |
| 19 | 5204122351983254700 |
| 20 | 120139520273298100575 |
| 21 | 2903216115946088267325 |
[1,1,3,12,63,405,3075,26880,265545,2922885, 35447895,469396620,6736095135,104102463465, 1723322736135,30416726340000,570089983287825, 11306156398562025,236514323713142475, 5204122351983254700,120139520273298100575, 2903216115946088267325] |