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 | 0 |
| 1 | 1 |
| 2 | -1 |
| 3 | -1 |
| 4 | 0 |
| 5 | -11 |
| 6 | 15 |
| 7 | 27 |
| 8 | -504 |
| 9 | 11817 |
| 10 | -94185 |
| 11 | 1226455 |
| 12 | -12442056 |
| 13 | 155936221 |
| 14 | -1995562569 |
| 15 | 27870901107 |
| 16 | -423463160400 |
| 17 | 6793396567633 |
| 18 | -117302680146033 |
| 19 | 2130615128588591 |
| 20 | -40960288523646320 |
| 21 | 827190717641773765 |