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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 18 |
| 3 | 87 |
| 4 | 431 |
| 5 | 2150 |
| 6 | 10744 |
| 7 | 53713 |
| 8 | 268557 |
| 9 | 1342776 |
| 10 | 6713870 |
| 11 | 33569339 |
| 12 | 167846683 |
| 13 | 839233402 |
| 14 | 4196166996 |
| 15 | 20980834965 |
| 16 | 104904174809 |
| 17 | 524520874028 |
| 18 | 2622604370122 |
| 19 | 13113021850591 |
| 20 | 65565109252935 |
| 21 | 327825546264654 |
[1,4,18,87,431,2150,10744,53713,268557,1342776, 6713870,33569339,167846683,839233402,4196166996, 20980834965,104904174809,524520874028, 2622604370122,13113021850591,65565109252935, 327825546264654] |