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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 42 |
| 4 | 648 |
| 5 | 13620 |
| 6 | 362520 |
| 7 | 11696160 |
| 8 | 443748480 |
| 9 | 19362566160 |
| 10 | 955384416000 |
| 11 | 52602602199840 |
| 12 | 3197371241877120 |
| 13 | 212668103188981440 |
| 14 | 15364762939152506880 |
| 15 | 1198224396270059558400 |
| 16 | 100322035878465399398400 |
| 17 | 8975632678570151712518400 |
| 18 | 854594260998625751469465600 |
[1,1,4,42,648,13620,362520,11696160,443748480, 19362566160,955384416000,52602602199840, 3197371241877120,212668103188981440, 15364762939152506880,1198224396270059558400, 100322035878465399398400, 8975632678570151712518400, 854594260998625751469465600] |