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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 21 |
| 3 | 109 |
| 4 | 575 |
| 5 | 3047 |
| 6 | 16192 |
| 7 | 86171 |
| 8 | 458972 |
| 9 | 2445825 |
| 10 | 13037508 |
| 11 | 69509558 |
| 12 | 370634265 |
| 13 | 1976421031 |
| 14 | 10539858413 |
| 15 | 56208784808 |
| 16 | 299766435277 |
| 17 | 1598704176149 |
| 18 | 8526238418811 |
| 19 | 45472594256564 |
| 20 | 242517996824355 |
[1,4,21,109,575,3047,16192,86171,458972,2445825, 13037508,69509558,370634265,1976421031, 10539858413,56208784808,299766435277, 1598704176149,8526238418811,45472594256564, 242517996824355] |