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| n | a(n) |
|---|---|
| 1 | 3 |
| 2 | 20 |
| 3 | 126 |
| 4 | 780 |
| 5 | 4808 |
| 6 | 29608 |
| 7 | 182288 |
| 8 | 1122240 |
| 9 | 6908896 |
| 10 | 42533440 |
| 11 | 261849728 |
| 12 | 1612032128 |
| 13 | 9924194048 |
| 14 | 61096565760 |
| 15 | 376130326016 |
| 16 | 2315580595200 |
| 17 | 14255467112448 |
| 18 | 87761291061248 |
| 19 | 540287045521408 |
| 20 | 3326182739886080 |
[3,20,126,780,4808,29608,182288,1122240,6908896, 42533440,261849728,1612032128,9924194048, 61096565760,376130326016,2315580595200, 14255467112448,87761291061248,540287045521408, 3326182739886080] |