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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 20 |
| 3 | 78 |
| 4 | 300 |
| 5 | 980 |
| 6 | 3120 |
| 7 | 9527 |
| 8 | 28560 |
| 9 | 84114 |
| 10 | 244750 |
| 11 | 704836 |
| 12 | 2013120 |
| 13 | 5709613 |
| 14 | 16097900 |
| 15 | 45155190 |
| 16 | 126099120 |
| 17 | 350765012 |
| 18 | 972333360 |
| 19 | 2687024099 |
| 20 | 7404969000 |
| 21 | 20355783546 |
| 22 | 55829853970 |
| 23 | 152808294148 |
| 24 | 417451104000 |
| 25 | 1138438728025 |
| 26 | 3099690135620 |
| 27 | 8427213134622 |
[0,1,20,78,300,980,3120,9527,28560,84114,244750, 704836,2013120,5709613,16097900,45155190, 126099120,350765012,972333360,2687024099, 7404969000,20355783546,55829853970,152808294148, 417451104000,1138438728025,3099690135620, 8427213134622] |