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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 16 |
| 4 | 71 |
| 5 | 351 |
| 6 | 1891 |
| 7 | 10888 |
| 8 | 66139 |
| 9 | 420235 |
| 10 | 2775981 |
| 11 | 18978873 |
| 12 | 133828922 |
| 13 | 970678790 |
| 14 | 7226115267 |
| 15 | 55115404005 |
| 16 | 430080085093 |
| 17 | 3429311454089 |
| 18 | 27912555377062 |
| 19 | 231710034354364 |
| 20 | 1960247357996533 |
| 21 | 16889105788701591 |
[1,1,4,16,71,351,1891,10888,66139,420235,2775981, 18978873,133828922,970678790,7226115267, 55115404005,430080085093,3429311454089, 27912555377062,231710034354364,1960247357996533, 16889105788701591] |