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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 16 |
| 3 | 66 |
| 4 | 292 |
| 5 | 1280 |
| 6 | 5644 |
| 7 | 24782 |
| 8 | 108940 |
| 9 | 478862 |
| 10 | 2104866 |
| 11 | 9252000 |
| 12 | 40667620 |
| 13 | 178756504 |
| 14 | 785732636 |
| 15 | 3453724976 |
| 16 | 15181011874 |
| 17 | 66728857148 |
| 18 | 293309853452 |
| 19 | 1289257359676 |
| 20 | 5666991817956 |
| 21 | 24909531073894 |
| 22 | 109491024206644 |
[4,16,66,292,1280,5644,24782,108940,478862, 2104866,9252000,40667620,178756504,785732636, 3453724976,15181011874,66728857148,293309853452, 1289257359676,5666991817956,24909531073894, 109491024206644] |