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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 112 |
| 4 | 664 |
| 5 | 4064 |
| 6 | 25376 |
| 7 | 160640 |
| 8 | 1027168 |
| 9 | 6618496 |
| 10 | 42904960 |
| 11 | 279503360 |
| 12 | 1828222720 |
| 13 | 11999226880 |
| 14 | 78984381440 |
| 15 | 521218322432 |
| 16 | 3447059138048 |
| 17 | 22840932997120 |
| 18 | 151607254267904 |
| 19 | 1007830488424448 |
| 20 | 6708862677274624 |
[1,4,20,112,664,4064,25376,160640,1027168,6618496, 42904960,279503360,1828222720,11999226880, 78984381440,521218322432,3447059138048, 22840932997120,151607254267904,1007830488424448, 6708862677274624] |