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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 24 |
| 4 | 112 |
| 5 | 576 |
| 6 | 3180 |
| 7 | 18540 |
| 8 | 112840 |
| 9 | 711016 |
| 10 | 4610036 |
| 11 | 30614932 |
| 12 | 207498060 |
| 13 | 1431260984 |
| 14 | 10024424284 |
| 15 | 71158769844 |
| 16 | 511160782120 |
| 17 | 3710968813976 |
| 18 | 27198487483336 |
| 19 | 201060296071624 |
| 20 | 1497907249045496 |
| 21 | 11238798692083888 |
[1,2,6,24,112,576,3180,18540,112840,711016, 4610036,30614932,207498060,1431260984,10024424284, 71158769844,511160782120,3710968813976, 27198487483336,201060296071624,1497907249045496, 11238798692083888] |