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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 26 |
| 3 | 139 |
| 4 | 726 |
| 5 | 3774 |
| 6 | 19601 |
| 7 | 101784 |
| 8 | 528526 |
| 9 | 2744419 |
| 10 | 14250626 |
| 11 | 73997554 |
| 12 | 384238401 |
| 13 | 1995189564 |
| 14 | 10360186226 |
| 15 | 53796120699 |
| 16 | 279340789726 |
| 17 | 1450500069334 |
| 18 | 7531841136401 |
| 19 | 39109705751344 |
| 20 | 203080369893126 |
[1,4,26,139,726,3774,19601,101784,528526,2744419, 14250626,73997554,384238401,1995189564, 10360186226,53796120699,279340789726, 1450500069334,7531841136401,39109705751344, 203080369893126] |