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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 16 |
| 3 | 68 |
| 4 | 272 |
| 5 | 1092 |
| 6 | 4624 |
| 7 | 18500 |
| 8 | 74512 |
| 9 | 314436 |
| 10 | 1258512 |
| 11 | 5083204 |
| 12 | 21382416 |
| 13 | 85627972 |
| 14 | 346707472 |
| 15 | 1454102596 |
| 16 | 5826897680 |
| 17 | 23643380804 |
| 18 | 98889463824 |
| 19 | 396564832324 |
| 20 | 1612065835280 |
| 21 | 6725490517060 |
| 22 | 26992215104016 |
[1,4,16,68,272,1092,4624,18500,74512,314436, 1258512,5083204,21382416,85627972,346707472, 1454102596,5826897680,23643380804,98889463824, 396564832324,1612065835280,6725490517060, 26992215104016] |