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 | 7 |
| 2 | 15 |
| 3 | 63 |
| 4 | 255 |
| 5 | 4095 |
| 6 | 16383 |
| 7 | 262143 |
| 8 | 1048575 |
| 9 | 16777215 |
| 10 | 1073741823 |
| 11 | 4294967295 |
| 12 | 274877906943 |
| 13 | 4398046511103 |
| 14 | 17592186044415 |
| 15 | 281474976710655 |
| 16 | 18014398509481983 |
| 17 | 1152921504606846975 |
| 18 | 4611686018427387903 |
| 19 | 295147905179352825855 |
| 20 | 4722366482869645213695 |