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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 2 |
| 3 | 12 |
| 4 | 84 |
| 5 | 640 |
| 6 | 5430 |
| 7 | 50988 |
| 8 | 526568 |
| 9 | 5940576 |
| 10 | 72755370 |
| 11 | 961839340 |
| 12 | 13656650172 |
| 13 | 207316760352 |
| 14 | 3351430059614 |
| 15 | 57487448630220 |
| 16 | 1042952206111440 |
| 17 | 19954639072648768 |
| 18 | 401578933206288978 |
| 19 | 8480263630552747596 |
| 20 | 187505565234912994340 |
| 21 | 4332318322289242716480 |
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