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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 12 |
| 3 | 42 |
| 4 | 148 |
| 5 | 540 |
| 6 | 1990 |
| 7 | 7434 |
| 8 | 27972 |
| 9 | 106008 |
| 10 | 403764 |
| 11 | 1544796 |
| 12 | 5931486 |
| 13 | 22846252 |
| 14 | 88228998 |
| 15 | 341518606 |
| 16 | 1324627044 |
| 17 | 5146959168 |
| 18 | 20030812360 |
| 19 | 78066774400 |
| 20 | 304643526276 |
| 21 | 1190209498344 |
| 22 | 4654949367204 |
| 23 | 18223301727108 |
[1,4,12,42,148,540,1990,7434,27972,106008,403764, 1544796,5931486,22846252,88228998,341518606, 1324627044,5146959168,20030812360,78066774400, 304643526276,1190209498344,4654949367204, 18223301727108] |