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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 18 |
| 5 | 90 |
| 6 | 540 |
| 7 | 3780 |
| 8 | 22680 |
| 9 | 181440 |
| 10 | 1814400 |
| 11 | 19958400 |
| 12 | 179625600 |
| 13 | 2335132800 |
| 14 | 32691859200 |
| 15 | 490377888000 |
| 16 | 5884534656000 |
| 17 | 100037089152000 |
| 18 | 1600593426432000 |
| 19 | 30411275102208000 |
| 20 | 456169126533120000 |
| 21 | 9579551657195520000 |
| 22 | 210750136458301440000 |
| 23 | 4847253138540933120000 |
[1,2,6,18,90,540,3780,22680,181440,1814400, 19958400,179625600,2335132800,32691859200, 490377888000,5884534656000,100037089152000, 1600593426432000,30411275102208000, 456169126533120000,9579551657195520000, 210750136458301440000,4847253138540933120000] |