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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 6 |
| 2 | 18 |
| 3 | 48 |
| 4 | 126 |
| 5 | 300 |
| 6 | 750 |
| 7 | 1686 |
| 8 | 4074 |
| 9 | 8868 |
| 10 | 20892 |
| 11 | 44634 |
| 12 | 103392 |
| 13 | 216348 |
| 14 | 499908 |
| 15 | 1017780 |
| 16 | 2383596 |
| 17 | 4648470 |
| 18 | 11271102 |
| 19 | 20763036 |
| 20 | 52671018 |
| 21 | 91377918 |
[1,6,18,48,126,300,750,1686,4074,8868,20892,44634, 103392,216348,499908,1017780,2383596,4648470, 11271102,20763036,52671018,91377918] |