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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 127 |
| 5 | 976 |
| 6 | 8776 |
| 7 | 90084 |
| 8 | 1037555 |
| 9 | 13233077 |
| 10 | 184956386 |
| 11 | 2809098986 |
| 12 | 46038214729 |
| 13 | 809411443790 |
| 14 | 15189361799522 |
| 15 | 302932433571356 |
| 16 | 6396529241755881 |
| 17 | 142523960797017589 |
| 18 | 3341115707515530400 |
| 19 | 82187749261419720712 |
| 20 | 2116421112495023612311 |
[1,1,4,20,127,976,8776,90084,1037555,13233077, 184956386,2809098986,46038214729,809411443790, 15189361799522,302932433571356,6396529241755881, 142523960797017589,3341115707515530400, 82187749261419720712,2116421112495023612311] |