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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 3 |
| 2 | 21 |
| 3 | 96 |
| 4 | 684 |
| 5 | 4320 |
| 6 | 35640 |
| 7 | 293760 |
| 8 | 2881440 |
| 9 | 29695680 |
| 10 | 345643200 |
| 11 | 4296499200 |
| 12 | 58518028800 |
| 13 | 851751936000 |
| 14 | 13338801100800 |
| 15 | 222145265664000 |
| 16 | 3934764739584000 |
| 17 | 73675580977152000 |
| 18 | 1455693852146688000 |
| 19 | 30238671742304256000 |
| 20 | 659012259039805440000 |
[1,3,21,96,684,4320,35640,293760,2881440,29695680, 345643200,4296499200,58518028800,851751936000, 13338801100800,222145265664000,3934764739584000, 73675580977152000,1455693852146688000, 30238671742304256000,659012259039805440000] |