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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 1 |
| 4 | 2 |
| 5 | 6 |
| 6 | 31 |
| 7 | 302 |
| 8 | 11706 |
| 9 | 3919531 |
| 10 | 47073195302 |
| 11 | 185055980038616706 |
| 12 | 8711901623180875318209094531 |
| 13 | 1612189902974799120179526719227635256224945302 |
[0,0,1,2,6,31,302,11706,3919531,47073195302, 185055980038616706,8711901623180875318209094531, 1612189902974799120179526719227635256224945302] |