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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 16 |
| 3 | 60 |
| 4 | 214 |
| 5 | 736 |
| 6 | 2464 |
| 7 | 8082 |
| 8 | 26092 |
| 9 | 83188 |
| 10 | 262582 |
| 11 | 822144 |
| 12 | 2557120 |
| 13 | 7910050 |
| 14 | 24357580 |
| 15 | 74720436 |
| 16 | 228485158 |
| 17 | 696795712 |
| 18 | 2120116768 |
| 19 | 6438264498 |
| 20 | 19518937516 |
| 21 | 59091592564 |
| 22 | 178675498006 |
| 23 | 539694923520 |
| 24 | 1628691435904 |
| 25 | 4911230068546 |
| 26 | 14799559211404 |
[1,4,16,60,214,736,2464,8082,26092,83188,262582, 822144,2557120,7910050,24357580,74720436, 228485158,696795712,2120116768,6438264498, 19518937516,59091592564,178675498006,539694923520, 1628691435904,4911230068546,14799559211404] |