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) |
|---|---|
| 1 | 15 |
| 2 | 4095 |
| 3 | 1152921504606846975 |
| 4 | 7237005577332262213973186563042994240829374041602535252466099000494570602495 |