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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 30 |
| 4 | 336 |
| 5 | 5100 |
| 6 | 97920 |
| 7 | 2275560 |
| 8 | 62092800 |
| 9 | 1946064960 |
| 10 | 68894582400 |
| 11 | 2719175659200 |
| 12 | 118394186803200 |
| 13 | 5637541432723200 |
| 14 | 291441967584768000 |
| 15 | 16256535509069856000 |
| 16 | 973201947936970752000 |
| 17 | 62239242631037010739200 |
| 18 | 4234925802836604395520000 |
[1,1,4,30,336,5100,97920,2275560,62092800, 1946064960,68894582400,2719175659200, 118394186803200,5637541432723200, 291441967584768000,16256535509069856000, 973201947936970752000,62239242631037010739200, 4234925802836604395520000] |