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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 1 |
| 3 | 4 |
| 4 | 10 |
| 5 | 56 |
| 6 | 304 |
| 7 | 1956 |
| 8 | 14856 |
| 9 | 130008 |
| 10 | 1237440 |
| 11 | 13084540 |
| 12 | 151656128 |
| 13 | 1907139360 |
| 14 | 25850635872 |
| 15 | 377651957184 |
| 16 | 5861540577920 |
| 17 | 97217787650368 |
| 18 | 1706293248844992 |
| 19 | 31680915068666736 |
| 20 | 618994402356397440 |
[1,1,4,10,56,304,1956,14856,130008,1237440, 13084540,151656128,1907139360,25850635872, 377651957184,5861540577920,97217787650368, 1706293248844992,31680915068666736, 618994402356397440] |