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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 2 |
| 3 | 6 |
| 4 | 42 |
| 5 | 305 |
| 6 | 2815 |
| 7 | 29792 |
| 8 | 362432 |
| 9 | 4952481 |
| 10 | 75239143 |
| 11 | 1257202584 |
| 12 | 22918653428 |
| 13 | 452620972245 |
| 14 | 9626556838015 |
| 15 | 219367419292972 |
| 16 | 5332164894151648 |
| 17 | 137709755844024929 |
| 18 | 3765736630207259055 |
| 19 | 108696751776637007080 |
| 20 | 3302628833563666988740 |
[1,0,2,6,42,305,2815,29792,362432,4952481, 75239143,1257202584,22918653428,452620972245, 9626556838015,219367419292972,5332164894151648, 137709755844024929,3765736630207259055, 108696751776637007080,3302628833563666988740] |