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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 6 |
| 3 | 72 |
| 4 | 1254 |
| 5 | 28794 |
| 6 | 819888 |
| 7 | 27869316 |
| 8 | 1101032100 |
| 9 | 49570797780 |
| 10 | 2505156062472 |
| 11 | 140417898936336 |
| 12 | 8644973807845368 |
| 13 | 579908437058338920 |
| 14 | 42098286646367326368 |
| 15 | 3288252917244250703664 |
| 16 | 274974019392668843164176 |
| 17 | 24510436934573885695407504 |
| 18 | 2319947117871178825560902112 |
[1,1,6,72,1254,28794,819888,27869316,1101032100, 49570797780,2505156062472,140417898936336, 8644973807845368,579908437058338920, 42098286646367326368,3288252917244250703664, 274974019392668843164176, 24510436934573885695407504, 2319947117871178825560902112] |