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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 24 |
| 3 | 168 |
| 4 | 1284 |
| 5 | 10384 |
| 6 | 87360 |
| 7 | 756704 |
| 8 | 6703168 |
| 9 | 60444928 |
| 10 | 552990592 |
| 11 | 5120101760 |
| 12 | 47887472000 |
| 13 | 451759449600 |
| 14 | 4293634467840 |
| 15 | 41073654689280 |
| 16 | 395170166443008 |
| 17 | 3821262491103232 |
| 18 | 37118973530660864 |
| 19 | 362035991963869184 |
| 20 | 3544080121528001536 |
[1,4,24,168,1284,10384,87360,756704,6703168, 60444928,552990592,5120101760,47887472000, 451759449600,4293634467840,41073654689280, 395170166443008,3821262491103232, 37118973530660864,362035991963869184, 3544080121528001536] |