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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 132 |
| 5 | 900 |
| 6 | 10560 |
| 7 | 96600 |
| 8 | 1500240 |
| 9 | 19066320 |
| 10 | 369714240 |
| 11 | 5359163040 |
| 12 | 147177898560 |
| 13 | 2443958637120 |
| 14 | 76298578836480 |
| 15 | 1621294897622400 |
| 16 | 58906376034105600 |
| 17 | 1309870975014201600 |
| 18 | 60357698670132864000 |
| 19 | 1469955465552513139200 |
| 20 | 74262907856067567436800 |
[1,1,4,18,132,900,10560,96600,1500240,19066320, 369714240,5359163040,147177898560,2443958637120, 76298578836480,1621294897622400,58906376034105600, 1309870975014201600,60357698670132864000, 1469955465552513139200,74262907856067567436800] |