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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 104 |
| 4 | 592 |
| 5 | 3592 |
| 6 | 23212 |
| 7 | 158652 |
| 8 | 1141964 |
| 9 | 8621820 |
| 10 | 68063300 |
| 11 | 560324488 |
| 12 | 4799515756 |
| 13 | 42690098440 |
| 14 | 393605305676 |
| 15 | 3755794383536 |
| 16 | 37034899306132 |
| 17 | 376877989985712 |
| 18 | 3953002335087184 |
| 19 | 42686126230197500 |
| 20 | 474037727935110620 |
[1,4,20,104,592,3592,23212,158652,1141964,8621820, 68063300,560324488,4799515756,42690098440, 393605305676,3755794383536,37034899306132, 376877989985712,3953002335087184, 42686126230197500,474037727935110620] |