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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 1 |
| 4 | 2 |
| 5 | 8 |
| 6 | 28 |
| 7 | 94 |
| 8 | 304 |
| 9 | 960 |
| 10 | 2976 |
| 11 | 9096 |
| 12 | 27488 |
| 13 | 82304 |
| 14 | 244544 |
| 15 | 721888 |
| 16 | 2119168 |
| 17 | 6191104 |
| 18 | 18011136 |
| 19 | 52203648 |
| 20 | 150809088 |
| 21 | 434382848 |
| 22 | 1247857664 |
| 23 | 3576143360 |
| 24 | 10226274304 |
| 25 | 29184704512 |
| 26 | 83138240512 |
| 27 | 236438194176 |
| 28 | 671370043392 |
[0,0,1,2,8,28,94,304,960,2976,9096,27488,82304, 244544,721888,2119168,6191104,18011136,52203648, 150809088,434382848,1247857664,3576143360, 10226274304,29184704512,83138240512,236438194176, 671370043392] |