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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 4 |
| 3 | 13 |
| 4 | 54 |
| 5 | 278 |
| 6 | 1704 |
| 7 | 12126 |
| 8 | 98280 |
| 9 | 893904 |
| 10 | 9017280 |
| 11 | 99918120 |
| 12 | 1206500400 |
| 13 | 15768729360 |
| 14 | 221792780160 |
| 15 | 3340515069360 |
| 16 | 53641756586880 |
| 17 | 914849722725120 |
| 18 | 16514863528665600 |
| 19 | 314599179867396480 |
| 20 | 6306817346711481600 |
| 21 | 132727279189258656000 |
[1,2,4,13,54,278,1704,12126,98280,893904,9017280, 99918120,1206500400,15768729360,221792780160, 3340515069360,53641756586880,914849722725120, 16514863528665600,314599179867396480, 6306817346711481600,132727279189258656000] |