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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 2 |
| 3 | 6 |
| 4 | 54 |
| 5 | 460 |
| 6 | 3890 |
| 7 | 42364 |
| 8 | 512806 |
| 9 | 6698724 |
| 10 | 98496252 |
| 11 | 1585046584 |
| 12 | 27568171818 |
| 13 | 520043947020 |
| 14 | 10550553510016 |
| 15 | 228796551051436 |
| 16 | 5291441028244966 |
| 17 | 129967582592816500 |
| 18 | 3377869204044947060 |
| 19 | 92652519380506887784 |
| 20 | 2674716530794339146244 |
[0,0,2,6,54,460,3890,42364,512806,6698724, 98496252,1585046584,27568171818,520043947020, 10550553510016,228796551051436,5291441028244966, 129967582592816500,3377869204044947060, 92652519380506887784,2674716530794339146244] |