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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 6 |
| 2 | 18 |
| 3 | 132 |
| 4 | 810 |
| 5 | 5724 |
| 6 | 42156 |
| 7 | 323352 |
| 8 | 2550042 |
| 9 | 20559660 |
| 10 | 168680196 |
[1,6,18,132,810,5724,42156,323352,2550042, 20559660,168680196] |