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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 5 |
| 3 | 20 |
| 4 | 104 |
| 5 | 632 |
| 6 | 4396 |
| 7 | 34680 |
| 8 | 307236 |
| 9 | 3026472 |
| 10 | 32849364 |
| 11 | 389704800 |
| 12 | 5017492320 |
| 13 | 69678231552 |
| 14 | 1038078389376 |
| 15 | 16513758904320 |
| 16 | 279354776803200 |
| 17 | 5007072973075200 |
| 18 | 94783054774919040 |
| 19 | 1889504358498754560 |
| 20 | 39565281716813111040 |
| 21 | 868194780280625779200 |
[1,2,5,20,104,632,4396,34680,307236,3026472, 32849364,389704800,5017492320,69678231552, 1038078389376,16513758904320,279354776803200, 5007072973075200,94783054774919040, 1889504358498754560,39565281716813111040, 868194780280625779200] |