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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 18 |
| 4 | 56 |
| 5 | 176 |
| 6 | 558 |
| 7 | 1778 |
| 8 | 5686 |
| 9 | 18230 |
| 10 | 58558 |
| 11 | 188366 |
| 12 | 606588 |
| 13 | 1955044 |
| 14 | 6305418 |
| 15 | 20347342 |
| 16 | 65689088 |
| 17 | 212146400 |
| 18 | 685342218 |
| 19 | 2214556478 |
| 20 | 7157409064 |
| 21 | 23136645472 |
| 22 | 74801223162 |
| 23 | 241863933094 |
| 24 | 782131232390 |
| 25 | 2529458676326 |
[1,2,6,18,56,176,558,1778,5686,18230,58558,188366, 606588,1955044,6305418,20347342,65689088, 212146400,685342218,2214556478,7157409064, 23136645472,74801223162,241863933094,782131232390, 2529458676326] |