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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 12 |
| 4 | 40 |
| 5 | 136 |
| 6 | 460 |
| 7 | 1556 |
| 8 | 5264 |
| 9 | 17808 |
| 10 | 60244 |
| 11 | 203804 |
| 12 | 689464 |
| 13 | 2332440 |
| 14 | 7890588 |
| 15 | 26693668 |
| 16 | 90304032 |
| 17 | 305496352 |
| 18 | 1033486756 |
| 19 | 3496260652 |
| 20 | 11827765064 |
| 21 | 40013042600 |
| 22 | 135363153516 |
| 23 | 457930268212 |
| 24 | 1549167000752 |
| 25 | 5240794423984 |
[1,4,12,40,136,460,1556,5264,17808,60244,203804, 689464,2332440,7890588,26693668,90304032, 305496352,1033486756,3496260652,11827765064, 40013042600,135363153516,457930268212, 1549167000752,5240794423984] |