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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 25 |
| 5 | 125 |
| 6 | 750 |
| 7 | 5251 |
| 8 | 42008 |
| 9 | 378072 |
| 10 | 3780721 |
| 11 | 41587931 |
| 12 | 499055172 |
| 13 | 6487717237 |
| 14 | 90828041318 |
| 15 | 1362420619770 |
| 16 | 21798729916321 |
| 17 | 370578408577457 |
| 18 | 6670411354394226 |
| 19 | 126737815733490295 |
| 20 | 2534756314669805900 |
| 21 | 53229882608065923900 |
| 22 | 1171057417377450325801 |
[0,1,2,6,25,125,750,5251,42008,378072,3780721, 41587931,499055172,6487717237,90828041318, 1362420619770,21798729916321,370578408577457, 6670411354394226,126737815733490295, 2534756314669805900,53229882608065923900, 1171057417377450325801] |