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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 3 |
| 2 | 8 |
| 3 | 28 |
| 4 | 124 |
| 5 | 668 |
| 6 | 4248 |
| 7 | 31176 |
| 8 | 259488 |
| 9 | 2416032 |
| 10 | 24886080 |
| 11 | 281004480 |
| 12 | 3451887360 |
| 13 | 45832538880 |
| 14 | 654109585920 |
| 15 | 9986000371200 |
| 16 | 162391354675200 |
| 17 | 2802498609254400 |
| 18 | 51156349822771200 |
| 19 | 984775394044108800 |
| 20 | 19938798081699840000 |
| 21 | 423580563732049920000 |
[1,3,8,28,124,668,4248,31176,259488,2416032, 24886080,281004480,3451887360,45832538880, 654109585920,9986000371200,162391354675200, 2802498609254400,51156349822771200, 984775394044108800,19938798081699840000, 423580563732049920000] |