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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 3 |
| 2 | 20 |
| 3 | 154 |
| 4 | 1260 |
| 5 | 10659 |
| 6 | 92092 |
| 7 | 807300 |
| 8 | 7152444 |
| 9 | 63882940 |
| 10 | 574221648 |
| 11 | 5188082354 |
| 12 | 47073334100 |
| 13 | 428634152730 |
| 14 | 3914819231400 |
| 15 | 35848190542920 |
| 16 | 329007937216860 |
| 17 | 3025582795190340 |
| 18 | 27872496751392496 |
| 19 | 257172019222240200 |
| 20 | 2376196095585231920 |
| 21 | 21983235825545286435 |
[1,3,20,154,1260,10659,92092,807300,7152444, 63882940,574221648,5188082354,47073334100, 428634152730,3914819231400,35848190542920, 329007937216860,3025582795190340, 27872496751392496,257172019222240200, 2376196095585231920,21983235825545286435] |