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 | 31 |
| 2 | 255 |
| 3 | 2047 |
| 4 | 16383 |
| 5 | 131071 |
| 6 | 1048575 |
| 7 | 8388607 |
| 8 | 67108863 |
| 9 | 536870911 |
| 10 | 4294967295 |
| 11 | 34359738367 |
| 12 | 274877906943 |
| 13 | 2199023255551 |
| 14 | 17592186044415 |
| 15 | 140737488355327 |
| 16 | 1125899906842623 |
| 17 | 9007199254740991 |
| 18 | 72057594037927935 |
| 19 | 576460752303423487 |
| 20 | 4611686018427387903 |