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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 2 |
| 3 | 4 |
| 4 | 18 |
| 5 | 50 |
| 6 | 176 |
| 7 | 546 |
| 8 | 1806 |
| 9 | 5780 |
| 10 | 18810 |
| 11 | 60698 |
| 12 | 196704 |
| 13 | 636090 |
| 14 | 2059174 |
| 15 | 6662420 |
| 16 | 21562002 |
| 17 | 69772930 |
| 18 | 225795088 |
| 19 | 730679922 |
| 20 | 2364543390 |
| 21 | 7651801300 |
| 22 | 24761784522 |
| 23 | 80130760714 |
| 24 | 259308681408 |
| 25 | 839140370250 |
| 26 | 2715515523446 |
[0,0,2,4,18,50,176,546,1806,5780,18810,60698, 196704,636090,2059174,6662420,21562002,69772930, 225795088,730679922,2364543390,7651801300, 24761784522,80130760714,259308681408,839140370250, 2715515523446] |