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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 91 |
| 4 | 410 |
| 5 | 1845 |
| 6 | 8303 |
| 7 | 37366 |
| 8 | 168151 |
| 9 | 756680 |
| 10 | 3405062 |
| 11 | 15322783 |
| 12 | 68952523 |
| 13 | 310286355 |
| 14 | 1396288601 |
| 15 | 6283298708 |
| 16 | 28274844190 |
| 17 | 127236798856 |
| 18 | 572565594852 |
| 19 | 2576545176835 |
| 20 | 11594453295761 |
| 21 | 52175039830928 |
| 22 | 234787679239180 |
[1,4,20,91,410,1845,8303,37366,168151,756680, 3405062,15322783,68952523,310286355,1396288601, 6283298708,28274844190,127236798856,572565594852, 2576545176835,11594453295761,52175039830928, 234787679239180] |