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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 20 |
| 4 | 68 |
| 5 | 240 |
| 6 | 864 |
| 7 | 3152 |
| 8 | 11616 |
| 9 | 43136 |
| 10 | 161152 |
| 11 | 604992 |
| 12 | 2280416 |
| 13 | 8624832 |
| 14 | 32714688 |
| 15 | 124399488 |
| 16 | 474066560 |
| 17 | 1810053120 |
| 18 | 6922776576 |
| 19 | 26517173760 |
| 20 | 101710338048 |
| 21 | 390603984896 |
| 22 | 1501732753408 |
| 23 | 5779500226560 |
| 24 | 22263437981184 |
| 25 | 85835254221824 |
| 26 | 331193445626880 |
[1,2,6,20,68,240,864,3152,11616,43136,161152, 604992,2280416,8624832,32714688,124399488, 474066560,1810053120,6922776576,26517173760, 101710338048,390603984896,1501732753408, 5779500226560,22263437981184,85835254221824, 331193445626880] |