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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 2 |
| 4 | 6 |
| 5 | 20 |
| 6 | 74 |
| 7 | 300 |
| 8 | 1302 |
| 9 | 5944 |
| 10 | 28266 |
| 11 | 139010 |
| 12 | 703102 |
| 13 | 3641956 |
| 14 | 19255106 |
| 15 | 103630920 |
| 16 | 566522778 |
| 17 | 3140130354 |
| 18 | 17620845976 |
| 19 | 99977635264 |
| 20 | 572935630884 |
| 21 | 3313078283974 |
[0,1,2,6,20,74,300,1302,5944,28266,139010,703102, 3641956,19255106,103630920,566522778,3140130354, 17620845976,99977635264,572935630884, 3313078283974] |