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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 2 |
| 4 | 3 |
| 5 | 6 |
| 6 | 15 |
| 7 | 42 |
| 8 | 123 |
| 9 | 380 |
| 10 | 1212 |
| 11 | 3966 |
| 12 | 13265 |
| 13 | 45144 |
| 14 | 155955 |
| 15 | 545690 |
| 16 | 1930635 |
| 17 | 6897210 |
| 18 | 24852576 |
| 19 | 90237582 |
| 20 | 329896569 |
| 21 | 1213528736 |
| 22 | 4489041219 |
| 23 | 16690581534 |
| 24 | 62346895571 |
| 25 | 233893503330 |
| 26 | 880918093866 |
| 27 | 3329949535934 |
[0,0,2,3,6,15,42,123,380,1212,3966,13265,45144, 155955,545690,1930635,6897210,24852576,90237582, 329896569,1213528736,4489041219,16690581534, 62346895571,233893503330,880918093866, 3329949535934] |