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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 0 |
| 3 | 6 |
| 4 | 12 |
| 5 | 40 |
| 6 | 900 |
| 7 | 8568 |
| 8 | 80640 |
| 9 | 1170720 |
| 10 | 19625760 |
| 11 | 335079360 |
| 12 | 6259524480 |
| 13 | 132897697920 |
| 14 | 3078950434560 |
| 15 | 75934287475200 |
| 16 | 2010354982272000 |
| 17 | 57241482249369600 |
| 18 | 1735591000023336960 |
| 19 | 55694476130213652480 |
| 20 | 1889613850762113638400 |
[1,0,0,6,12,40,900,8568,80640,1170720,19625760, 335079360,6259524480,132897697920,3078950434560, 75934287475200,2010354982272000,57241482249369600, 1735591000023336960,55694476130213652480, 1889613850762113638400] |