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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 6 |
| 3 | 42 |
| 4 | 288 |
| 5 | 1992 |
| 6 | 13980 |
| 7 | 99432 |
| 8 | 715344 |
| 9 | 5196336 |
| 10 | 38056284 |
| 11 | 280658100 |
| 12 | 2082218160 |
| 13 | 15528409920 |
| 14 | 116331315360 |
| 15 | 874985339760 |
| 16 | 6604555554720 |
| 17 | 50010373864416 |
| 18 | 379760762209692 |
| 19 | 2891169309592548 |
| 20 | 22062102167330592 |
[0,6,42,288,1992,13980,99432,715344,5196336, 38056284,280658100,2082218160,15528409920, 116331315360,874985339760,6604555554720, 50010373864416,379760762209692,2891169309592548, 22062102167330592] |