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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 4 |
| 3 | 16 |
| 4 | 84 |
| 5 | 496 |
| 6 | 3120 |
| 7 | 20416 |
| 8 | 137300 |
| 9 | 942384 |
| 10 | 6572336 |
| 11 | 46432960 |
| 12 | 331580272 |
| 13 | 2389352256 |
| 14 | 17351364160 |
| 15 | 126851634432 |
| 16 | 932823545428 |
| 17 | 6895102385072 |
| 18 | 51199649648048 |
| 19 | 381738099675840 |
| 20 | 2856639909232112 |
| 21 | 21447771308542784 |
[1,4,4,16,84,496,3120,20416,137300,942384,6572336, 46432960,331580272,2389352256,17351364160, 126851634432,932823545428,6895102385072, 51199649648048,381738099675840,2856639909232112, 21447771308542784] |