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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 110 |
| 4 | 638 |
| 5 | 3828 |
| 6 | 23515 |
| 7 | 146974 |
| 8 | 930940 |
| 9 | 5959555 |
| 10 | 38485153 |
| 11 | 250361331 |
| 12 | 1639087576 |
| 13 | 10791380549 |
| 14 | 71409038308 |
| 15 | 474737253262 |
| 16 | 3169904556865 |
| 17 | 21253752401187 |
| 18 | 143071102097208 |
| 19 | 966819122581869 |
| 20 | 6558130910319694 |
| 21 | 44650977592122241 |
| 22 | 305125605291293360 |
| 23 | 2092700240205257834 |
[1,4,20,110,638,3828,23515,146974,930940,5959555, 38485153,250361331,1639087576,10791380549, 71409038308,474737253262,3169904556865, 21253752401187,143071102097208,966819122581869, 6558130910319694,44650977592122241, 305125605291293360,2092700240205257834] |