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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 15 |
| 3 | 56 |
| 4 | 207 |
| 5 | 764 |
| 6 | 2819 |
| 7 | 10400 |
| 8 | 38367 |
| 9 | 141540 |
| 10 | 522155 |
| 11 | 1926280 |
| 12 | 7106231 |
| 13 | 26215564 |
| 14 | 96711715 |
| 15 | 356778736 |
| 16 | 1316190767 |
| 17 | 4855553204 |
| 18 | 17912598619 |
| 19 | 66081283800 |
| 20 | 243780155047 |
| 21 | 899328229980 |
| 22 | 3317707567635 |
| 23 | 12239339472960 |
| 24 | 45152090014111 |
| 25 | 166570364123524 |
[1,4,15,56,207,764,2819,10400,38367,141540,522155, 1926280,7106231,26215564,96711715,356778736, 1316190767,4855553204,17912598619,66081283800, 243780155047,899328229980,3317707567635, 12239339472960,45152090014111,166570364123524] |