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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 18 |
| 3 | 92 |
| 4 | 536 |
| 5 | 3552 |
| 6 | 26608 |
| 7 | 223456 |
| 8 | 2085504 |
| 9 | 21450752 |
| 10 | 241320704 |
| 11 | 2949474816 |
| 12 | 38933066752 |
| 13 | 552141672448 |
| 14 | 8374148696064 |
| 15 | 135274709700608 |
| 16 | 2318995023429632 |
| 17 | 42051109758173184 |
| 18 | 804227474125029376 |
[1,4,18,92,536,3552,26608,223456,2085504,21450752, 241320704,2949474816,38933066752,552141672448, 8374148696064,135274709700608,2318995023429632, 42051109758173184,804227474125029376] |