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| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 1 |
| 3 | 4 |
| 4 | 19 |
| 5 | 92 |
| 6 | 468 |
| 7 | 2470 |
| 8 | 13394 |
| 9 | 74296 |
| 10 | 419463 |
| 11 | 2403502 |
| 12 | 13941202 |
| 13 | 81707264 |
| 14 | 483131437 |
| 15 | 2878697568 |
| 16 | 17267416917 |
| 17 | 104186446502 |
| 18 | 631913206482 |
| 19 | 3850548301808 |
| 20 | 23561351983192 |
| 21 | 144715120126370 |
[2,1,4,19,92,468,2470,13394,74296,419463,2403502, 13941202,81707264,483131437,2878697568, 17267416917,104186446502,631913206482, 3850548301808,23561351983192,144715120126370] |