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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 21 |
| 4 | 126 |
| 5 | 819 |
| 6 | 5611 |
| 7 | 39900 |
| 8 | 291719 |
| 9 | 2179181 |
| 10 | 16560175 |
| 11 | 127617168 |
| 12 | 994951887 |
| 13 | 7833555324 |
| 14 | 62196300997 |
| 15 | 497425570173 |
| 16 | 4003607595960 |
| 17 | 32404662671330 |
| 18 | 263586896132154 |
| 19 | 2153631763231319 |
| 20 | 17666722629907960 |
| 21 | 145449082369322208 |
| 22 | 1201414340736684702 |
[1,1,4,21,126,819,5611,39900,291719,2179181, 16560175,127617168,994951887,7833555324, 62196300997,497425570173,4003607595960, 32404662671330,263586896132154,2153631763231319, 17666722629907960,145449082369322208, 1201414340736684702] |