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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 2 |
| 3 | 4 |
| 4 | 12 |
| 5 | 40 |
| 6 | 116 |
| 7 | 252 |
| 8 | 768 |
| 9 | 2704 |
| 10 | 12756 |
| 11 | 63168 |
| 12 | 338308 |
| 13 | 1956876 |
| 14 | 11971780 |
| 15 | 77909884 |
| 16 | 535178056 |
| 17 | 3906154920 |
| 18 | 30146769464 |
| 19 | 244980749052 |
| 20 | 2094507398508 |
[1,2,4,12,40,116,252,768,2704,12756,63168,338308, 1956876,11971780,77909884,535178056,3906154920, 30146769464,244980749052,2094507398508] |