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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 36 |
| 5 | 240 |
| 6 | 2040 |
| 7 | 21000 |
| 8 | 235200 |
| 9 | 3265920 |
| 10 | 47628000 |
| 11 | 795175920 |
| 12 | 14411295360 |
| 13 | 279281882880 |
| 14 | 6049083680640 |
| 15 | 135398783520000 |
| 16 | 3367873821312000 |
| 17 | 87219462185856000 |
| 18 | 2403098895060864000 |
| 19 | 70681813184753971200 |
| 20 | 2138850761148310809600 |
[1,1,2,6,36,240,2040,21000,235200,3265920, 47628000,795175920,14411295360,279281882880, 6049083680640,135398783520000,3367873821312000, 87219462185856000,2403098895060864000, 70681813184753971200,2138850761148310809600] |