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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 116 |
| 5 | 1060 |
| 6 | 12702 |
| 7 | 187810 |
| 8 | 3296120 |
| 9 | 66897288 |
| 10 | 1540762010 |
| 11 | 39693752494 |
| 12 | 1130866726596 |
| 13 | 35300006582620 |
| 14 | 1198036854980630 |
| 15 | 43921652697277170 |
| 16 | 1729775120233353968 |
| 17 | 72831210167041246480 |
| 18 | 3264674481128340280242 |
| 19 | 155220967397580333229270 |
[0,1,4,18,116,1060,12702,187810,3296120,66897288, 1540762010,39693752494,1130866726596, 35300006582620,1198036854980630,43921652697277170, 1729775120233353968,72831210167041246480, 3264674481128340280242,155220967397580333229270] |