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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 138 |
| 5 | 1182 |
| 6 | 12166 |
| 7 | 146050 |
| 8 | 2003882 |
| 9 | 30930734 |
| 10 | 530477310 |
| 11 | 10007736906 |
| 12 | 205965058162 |
| 13 | 4592120925862 |
| 14 | 110259944144486 |
| 15 | 2836517343551714 |
| 16 | 77836238876829882 |
| 17 | 2269379773783175454 |
| 18 | 70057736432648552782 |
| 19 | 2282895953541692345722 |
[1,1,4,20,138,1182,12166,146050,2003882,30930734, 530477310,10007736906,205965058162,4592120925862, 110259944144486,2836517343551714, 77836238876829882,2269379773783175454, 70057736432648552782,2282895953541692345722] |