The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 4 |
| 3 | 25 |
| 4 | 408 |
| 5 | 18003 |
| 6 | 2289232 |
| 7 | 851979730 |
| 8 | 906147342695 |
| 9 | 2499603894160970 |
[2,4,25,408,18003,2289232,851979730,906147342695, 2499603894160970] |