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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 15 |
| 3 | 106 |
| 4 | 391 |
| 5 | 3991 |
| 6 | 30031 |
| 7 | 510511 |
| 8 | 9699691 |
| 9 | 223092871 |
| 10 | 6469693231 |
| 11 | 255887521891 |
| 12 | 7420738134811 |
| 13 | 304250263527211 |
| 14 | 13082761331670031 |
| 15 | 614889782588491411 |
| 16 | 32589158477190044731 |
| 17 | 1922760350154212639071 |
| 18 | 117288381359406970983271 |
| 19 | 7858321551080267055879091 |
| 20 | 557940830126698960967415391 |
[4,15,106,391,3991,30031,510511,9699691,223092871, 6469693231,255887521891,7420738134811, 304250263527211,13082761331670031, 614889782588491411,32589158477190044731, 1922760350154212639071,117288381359406970983271, 7858321551080267055879091, 557940830126698960967415391] |