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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 21 |
| 4 | 132 |
| 5 | 960 |
| 6 | 7920 |
| 7 | 73080 |
| 8 | 745920 |
| 9 | 8346240 |
| 10 | 101606400 |
| 11 | 1337212800 |
| 12 | 18920563200 |
| 13 | 286442956800 |
| 14 | 4620449433600 |
| 15 | 79114299264000 |
| 16 | 1433211107328000 |
| 17 | 27387931963392000 |
| 18 | 550604138692608000 |
| 19 | 11617107089043456000 |
| 20 | 256671161862635520000 |
| 21 | 5926549291918295040000 |
[1,4,21,132,960,7920,73080,745920,8346240, 101606400,1337212800,18920563200,286442956800, 4620449433600,79114299264000,1433211107328000, 27387931963392000,550604138692608000, 11617107089043456000,256671161862635520000, 5926549291918295040000] |