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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 2 |
| 3 | 4 |
| 4 | 12 |
| 5 | 40 |
| 6 | 152 |
| 7 | 640 |
| 8 | 2928 |
| 9 | 14400 |
| 10 | 75744 |
| 11 | 424640 |
| 12 | 2527552 |
| 13 | 15902848 |
| 14 | 105313408 |
| 15 | 731376640 |
| 16 | 5311088896 |
| 17 | 40233525248 |
| 18 | 317296341504 |
| 19 | 2600091120640 |
| 20 | 22099119279104 |
| 21 | 194487001540608 |
| 22 | 1769555559897088 |
| 23 | 16622286300921856 |
[1,0,2,4,12,40,152,640,2928,14400,75744,424640, 2527552,15902848,105313408,731376640,5311088896, 40233525248,317296341504,2600091120640, 22099119279104,194487001540608,1769555559897088, 16622286300921856] |