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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 4 |
| 2 | 20 |
| 3 | 76 |
| 4 | 252 |
| 5 | 764 |
| 6 | 2172 |
| 7 | 5884 |
| 8 | 15356 |
| 9 | 38908 |
| 10 | 96252 |
| 11 | 233468 |
| 12 | 557052 |
| 13 | 1310716 |
| 14 | 3047420 |
| 15 | 7012348 |
| 16 | 15990780 |
| 17 | 36175868 |
| 18 | 81264636 |
| 19 | 181403644 |
| 20 | 402653180 |
| 21 | 889192444 |
| 22 | 1954545660 |
| 23 | 4278190076 |
| 24 | 9328132092 |
| 25 | 20266876924 |
| 26 | 43889197052 |
[0,4,20,76,252,764,2172,5884,15356,38908,96252, 233468,557052,1310716,3047420,7012348,15990780, 36175868,81264636,181403644,402653180,889192444, 1954545660,4278190076,9328132092,20266876924, 43889197052] |