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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 18 |
| 5 | 60 |
| 6 | 204 |
| 7 | 720 |
| 8 | 2580 |
| 9 | 9408 |
| 10 | 34668 |
| 11 | 129024 |
| 12 | 483672 |
| 13 | 1824768 |
| 14 | 6919752 |
| 15 | 26357976 |
| 16 | 100778472 |
| 17 | 386591304 |
| 18 | 1487221056 |
| 19 | 5735785176 |
| 20 | 22170562656 |
| 21 | 85866332760 |
| 22 | 333151068528 |
| 23 | 1294658562912 |
| 24 | 5038469947992 |
| 25 | 19634243736168 |
[0,1,2,6,18,60,204,720,2580,9408,34668,129024, 483672,1824768,6919752,26357976,100778472, 386591304,1487221056,5735785176,22170562656, 85866332760,333151068528,1294658562912, 5038469947992,19634243736168] |