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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 96 |
| 5 | 600 |
| 6 | 4320 |
| 7 | 35280 |
| 8 | 322560 |
| 9 | 3265920 |
| 10 | 36288000 |
| 11 | 439084800 |
| 12 | 5748019200 |
| 13 | 80951270400 |
| 14 | 1220496076800 |
| 15 | 19615115520000 |
| 16 | 334764638208000 |
| 17 | 6046686277632000 |
| 18 | 115242726703104000 |
| 19 | 2311256907767808000 |
| 20 | 48658040163532800000 |
| 21 | 1072909785605898240000 |
[0,1,4,18,96,600,4320,35280,322560,3265920, 36288000,439084800,5748019200,80951270400, 1220496076800,19615115520000,334764638208000, 6046686277632000,115242726703104000, 2311256907767808000,48658040163532800000, 1072909785605898240000] |