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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 4 |
| 4 | 9 |
| 5 | 21 |
| 6 | 51 |
| 7 | 126 |
| 8 | 321 |
| 9 | 820 |
| 10 | 2160 |
| 11 | 5654 |
| 12 | 15272 |
| 13 | 40758 |
| 14 | 112280 |
| 15 | 304471 |
| 16 | 852164 |
| 17 | 2341980 |
| 18 | 6640755 |
| 19 | 18460066 |
| 20 | 52915999 |
[1,1,2,4,9,21,51,126,321,820,2160,5654,15272, 40758,112280,304471,852164,2341980,6640755, 18460066,52915999] |