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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 4 |
| 4 | 13 |
| 5 | 61 |
| 6 | 421 |
| 7 | 2626 |
| 8 | 27049 |
| 9 | 245953 |
| 10 | 3069721 |
| 11 | 40222216 |
| 12 | 576988501 |
| 13 | 10058716669 |
| 14 | 169773404893 |
| 15 | 3596206855606 |
| 16 | 73450508303761 |
| 17 | 1775382487932001 |
| 18 | 43993288886533489 |
| 19 | 1183551336464017708 |
| 20 | 34806599282992709341 |
| 21 | 1043452963148195577181 |
[1,1,1,4,13,61,421,2626,27049,245953,3069721, 40222216,576988501,10058716669,169773404893, 3596206855606,73450508303761,1775382487932001, 43993288886533489,1183551336464017708, 34806599282992709341,1043452963148195577181] |