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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 126 |
| 5 | 1160 |
| 6 | 15973 |
| 7 | 836021 |
| 8 | 1843120128 |
| 9 | 52989400714478 |
| 10 | 12418001077381302684 |
[1,1,4,18,126,1160,15973,836021,1843120128, 52989400714478,12418001077381302684] |