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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 70 |
| 5 | 273 |
| 6 | 1026 |
| 7 | 3876 |
| 8 | 14520 |
| 9 | 54477 |
| 10 | 204248 |
| 11 | 766935 |
| 12 | 2882934 |
| 13 | 10855425 |
| 14 | 40939888 |
| 15 | 154663938 |
| 16 | 585260496 |
| 17 | 2218309470 |
| 18 | 8421352956 |
| 19 | 32018897274 |
| 20 | 121918083980 |
| 21 | 464879984481 |
| 22 | 1774996216144 |
| 23 | 6785967883800 |
[0,1,4,18,70,273,1026,3876,14520,54477,204248, 766935,2882934,10855425,40939888,154663938, 585260496,2218309470,8421352956,32018897274, 121918083980,464879984481,1774996216144, 6785967883800] |