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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 116 |
| 4 | 724 |
| 5 | 4660 |
| 6 | 30356 |
| 7 | 198644 |
| 8 | 1302100 |
| 9 | 8540596 |
| 10 | 56031764 |
| 11 | 367636340 |
| 12 | 2412218836 |
| 13 | 15827788084 |
| 14 | 103854591380 |
| 15 | 681446713076 |
| 16 | 4471346955604 |
| 17 | 29338975191220 |
| 18 | 192509225431316 |
| 19 | 1263159425822324 |
| 20 | 8288287225499860 |
[1,4,20,116,724,4660,30356,198644,1302100,8540596, 56031764,367636340,2412218836,15827788084, 103854591380,681446713076,4471346955604, 29338975191220,192509225431316,1263159425822324, 8288287225499860] |