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| n | a(n) |
|---|---|
| 2 | 1 |
| 3 | 2 |
| 4 | 7 |
| 5 | 20 |
| 6 | 60 |
| 7 | 176 |
| 8 | 518 |
| 9 | 1520 |
| 10 | 4461 |
| 11 | 13090 |
| 12 | 38423 |
| 13 | 112828 |
| 14 | 331487 |
| 15 | 974442 |
| 16 | 2866125 |
| 17 | 8434992 |
| 18 | 24838275 |
| 19 | 73181142 |
| 20 | 215729781 |
| 21 | 636275820 |
| 22 | 1877569134 |
| 23 | 5543095404 |
| 24 | 16372140876 |
| 25 | 48377825216 |
| 26 | 143009973875 |
| 27 | 422918975726 |
| 28 | 1251154692297 |
[1,2,7,20,60,176,518,1520,4461,13090,38423,112828, 331487,974442,2866125,8434992,24838275,73181142, 215729781,636275820,1877569134,5543095404, 16372140876,48377825216,143009973875,422918975726, 1251154692297] |