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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 2 |
| 3 | 4 |
| 4 | 15 |
| 5 | 52 |
| 6 | 192 |
| 7 | 732 |
| 8 | 2853 |
| 9 | 11340 |
| 10 | 45756 |
| 11 | 186964 |
| 12 | 772057 |
| 13 | 3216912 |
| 14 | 13507936 |
| 15 | 57103516 |
| 16 | 242831553 |
| 17 | 1038056904 |
| 18 | 4458248878 |
| 19 | 19227639168 |
| 20 | 83239247342 |
| 21 | 361591141112 |
| 22 | 1575659134512 |
| 23 | 6885666506004 |
| 24 | 30169456622015 |
| 25 | 132506539685052 |
| 26 | 583278733961600 |
| 27 | 2572843291435872 |
[1,0,2,4,15,52,192,732,2853,11340,45756,186964, 772057,3216912,13507936,57103516,242831553, 1038056904,4458248878,19227639168,83239247342, 361591141112,1575659134512,6885666506004, 30169456622015,132506539685052,583278733961600, 2572843291435872] |