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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 128 |
| 4 | 1036 |
| 5 | 10308 |
| 6 | 122560 |
| 7 | 1701092 |
| 8 | 27053556 |
| 9 | 485683128 |
| 10 | 9723771156 |
| 11 | 214934627476 |
| 12 | 5201286731560 |
| 13 | 136818097071820 |
| 14 | 3888121468512308 |
| 15 | 118737900886653664 |
| 16 | 3878569457507036988 |
| 17 | 134960059001226137588 |
| 18 | 4984357865462772982112 |
[1,4,20,128,1036,10308,122560,1701092,27053556, 485683128,9723771156,214934627476,5201286731560, 136818097071820,3888121468512308, 118737900886653664,3878569457507036988, 134960059001226137588,4984357865462772982112] |