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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 9 |
| 4 | 60 |
| 5 | 540 |
| 6 | 6120 |
| 7 | 83790 |
| 8 | 1345680 |
| 9 | 24811920 |
| 10 | 516650400 |
| 11 | 11992503600 |
| 12 | 307069963200 |
| 13 | 8598348158400 |
| 14 | 261387760233600 |
| 15 | 8573572885878000 |
| 16 | 301809119163552000 |
| 17 | 11349727401396384000 |
| 18 | 454104511068656448000 |
| 19 | 19261139319649202976000 |
[0,1,2,9,60,540,6120,83790,1345680,24811920, 516650400,11992503600,307069963200,8598348158400, 261387760233600,8573572885878000, 301809119163552000,11349727401396384000, 454104511068656448000,19261139319649202976000] |