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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 15 |
| 4 | 80 |
| 5 | 490 |
| 6 | 3534 |
| 7 | 28938 |
| 8 | 266048 |
| 9 | 2710440 |
| 10 | 30311640 |
| 11 | 369127440 |
| 12 | 4862219592 |
| 13 | 68881435896 |
| 14 | 1044331262688 |
| 15 | 16872336545400 |
| 16 | 289380447338880 |
| 17 | 5251237965683520 |
| 18 | 100519388543098944 |
| 19 | 2024241909160239936 |
| 20 | 42780009017657888640 |
| 21 | 946724781741392908800 |
[1,1,4,15,80,490,3534,28938,266048,2710440, 30311640,369127440,4862219592,68881435896, 1044331262688,16872336545400,289380447338880, 5251237965683520,100519388543098944, 2024241909160239936,42780009017657888640, 946724781741392908800] |