This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
| n | a(n) |
|---|---|
| 0 | 3 |
| 1 | 7 |
| 2 | 21 |
| 3 | 73 |
| 4 | 273 |
| 5 | 1057 |
| 6 | 4161 |
| 7 | 16513 |
| 8 | 65793 |
| 9 | 262657 |
| 10 | 1049601 |
| 11 | 4196353 |
| 12 | 16781313 |
| 13 | 67117057 |
| 14 | 268451841 |
| 15 | 1073774593 |
| 16 | 4295032833 |
| 17 | 17180000257 |
| 18 | 68719738881 |
| 19 | 274878431233 |
| 20 | 1099512676353 |
| 21 | 4398048608257 |
| 22 | 17592190238721 |
| 23 | 70368752566273 |
| 24 | 281474993487873 |
| 25 | 1125899940397057 |