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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 0 |
| 4 | 3 |
| 5 | 4 |
| 6 | 16 |
| 7 | 42 |
| 8 | 137 |
| 9 | 426 |
| 10 | 1398 |
| 11 | 4622 |
| 12 | 15594 |
| 13 | 53252 |
| 14 | 184060 |
| 15 | 642392 |
| 16 | 2261829 |
| 17 | 8024726 |
| 18 | 28664946 |
| 19 | 103015222 |
| 20 | 372234190 |
| 21 | 1351655526 |
| 22 | 4930080182 |
| 23 | 18055480464 |
| 24 | 66371559466 |
| 25 | 244817099870 |
| 26 | 905883648170 |
| 27 | 3361795172242 |
| 28 | 12509691344838 |
[0,1,0,3,4,16,42,137,426,1398,4622,15594,53252, 184060,642392,2261829,8024726,28664946,103015222, 372234190,1351655526,4930080182,18055480464, 66371559466,244817099870,905883648170, 3361795172242,12509691344838] |