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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 2 |
| 3 | 6 |
| 4 | 22 |
| 5 | 100 |
| 6 | 548 |
| 7 | 3528 |
| 8 | 26136 |
| 9 | 219168 |
| 10 | 2053152 |
| 11 | 21257280 |
| 12 | 241087680 |
| 13 | 2972885760 |
| 14 | 39605518080 |
| 15 | 566931294720 |
| 16 | 8678326003200 |
| 17 | 141468564787200 |
| 18 | 2446811181158400 |
| 19 | 44753976117043200 |
| 20 | 863130293635276800 |
[0,0,2,6,22,100,548,3528,26136,219168,2053152, 21257280,241087680,2972885760,39605518080, 566931294720,8678326003200,141468564787200, 2446811181158400,44753976117043200, 863130293635276800] |