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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 21 |
| 4 | 144 |
| 5 | 1245 |
| 6 | 13140 |
| 7 | 164745 |
| 8 | 2399040 |
| 9 | 39834585 |
| 10 | 742940100 |
| 11 | 15374360925 |
| 12 | 349484058000 |
| 13 | 8654336615925 |
| 14 | 231842662751700 |
| 15 | 6679510641428625 |
| 16 | 205916703920928000 |
| 17 | 6762863294018456625 |
| 18 | 235719416966063530500 |
| 19 | 8689887736412502745125 |
[0,1,4,21,144,1245,13140,164745,2399040,39834585, 742940100,15374360925,349484058000,8654336615925, 231842662751700,6679510641428625, 205916703920928000,6762863294018456625, 235719416966063530500,8689887736412502745125] |