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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 24 |
| 3 | 144 |
| 4 | 864 |
| 5 | 5184 |
| 6 | 31104 |
| 7 | 186624 |
| 8 | 1119744 |
| 9 | 6718464 |
| 10 | 40310784 |
| 11 | 241864704 |
| 12 | 1451188224 |
| 13 | 8707129344 |
| 14 | 52242776064 |
| 15 | 313456656384 |
| 16 | 1880739938304 |
| 17 | 11284439629824 |
| 18 | 67706637778944 |
| 19 | 406239826673664 |
[1,4,24,144,864,5184,31104,186624,1119744,6718464, 40310784,241864704,1451188224,8707129344, 52242776064,313456656384,1880739938304, 11284439629824,67706637778944,406239826673664] |