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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 4 |
| 5 | 8 |
| 6 | 38 |
| 7 | 266 |
| 8 | 1680 |
| 9 | 10668 |
| 10 | 87444 |
| 11 | 1057140 |
| 12 | 10108644 |
| 13 | 114112320 |
| 14 | 1404689244 |
| 15 | 20654453736 |
| 16 | 294649177704 |
| 17 | 4556106886632 |
| 18 | 76389128377296 |
| 19 | 1363358922375072 |
| 20 | 25400563124152944 |
| 21 | 497084494996824840 |
[1,1,1,4,8,38,266,1680,10668,87444,1057140, 10108644,114112320,1404689244,20654453736, 294649177704,4556106886632,76389128377296, 1363358922375072,25400563124152944, 497084494996824840] |