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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 116 |
| 4 | 708 |
| 5 | 4452 |
| 6 | 28532 |
| 7 | 185300 |
| 8 | 1215268 |
| 9 | 8030404 |
| 10 | 53381844 |
| 11 | 356577588 |
| 12 | 2391430020 |
| 13 | 16092704292 |
| 14 | 108605848116 |
| 15 | 734783381652 |
| 16 | 4982063186916 |
| 17 | 33844621986180 |
| 18 | 230306722637204 |
| 19 | 1569571734301172 |
| 20 | 10711405584991300 |
[1,4,20,116,708,4452,28532,185300,1215268,8030404, 53381844,356577588,2391430020,16092704292, 108605848116,734783381652,4982063186916, 33844621986180,230306722637204,1569571734301172, 10711405584991300] |