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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 120 |
| 5 | 1100 |
| 6 | 13092 |
| 7 | 192360 |
| 8 | 3362128 |
| 9 | 68063760 |
| 10 | 1565077220 |
| 11 | 40275499264 |
| 12 | 1146522753288 |
| 13 | 35767031711424 |
| 14 | 1213309504953028 |
| 15 | 44464470056979840 |
| 16 | 1750597156059648544 |
| 17 | 73688248937813125376 |
| 18 | 3302340234016493521476 |
| 19 | 156980994639584475390976 |
| 20 | 7889413937606432287327000 |
[0,1,4,18,120,1100,13092,192360,3362128,68063760, 1565077220,40275499264,1146522753288, 35767031711424,1213309504953028,44464470056979840, 1750597156059648544,73688248937813125376, 3302340234016493521476,156980994639584475390976, 7889413937606432287327000] |