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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 4 |
| 4 | 20 |
| 5 | 108 |
| 6 | 678 |
| 7 | 4848 |
| 8 | 39264 |
| 9 | 355920 |
| 10 | 3575640 |
| 11 | 39454560 |
| 12 | 474501600 |
| 13 | 6178566240 |
| 14 | 86606881200 |
| 15 | 1300352981760 |
| 16 | 20821540239360 |
| 17 | 354184575816960 |
| 18 | 6378546460970880 |
| 19 | 121243261343500800 |
| 20 | 2425719783585369600 |
| 21 | 50955334461183398400 |
| 22 | 1121303792572973856000 |
| 23 | 25795667534014525593600 |
[0,1,4,20,108,678,4848,39264,355920,3575640, 39454560,474501600,6178566240,86606881200, 1300352981760,20821540239360,354184575816960, 6378546460970880,121243261343500800, 2425719783585369600,50955334461183398400, 1121303792572973856000,25795667534014525593600] |