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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 4 |
| 2 | 20 |
| 3 | 120 |
| 4 | 696 |
| 5 | 4060 |
| 6 | 23660 |
| 7 | 137904 |
| 8 | 803760 |
| 9 | 4684660 |
| 10 | 27304196 |
| 11 | 159140520 |
[0,4,20,120,696,4060,23660,137904,803760,4684660, 27304196,159140520] |