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 | 49 |
| 6 | -165 |
| 7 | -393 |
| 8 | -504 |
| 9 | 70785 |
| 10 | -434385 |
| 11 | -517265 |
| 12 | -12647976 |
| 13 | 654552625 |
| 14 | -6276987717 |
| 15 | 18362076327 |
| 16 | -636272644560 |
| 17 | 22683808563841 |
| 18 | -312594994151361 |
| 19 | 2817355930176479 |
| 20 | -72112751626318640 |
| 21 | 2222078368780722865 |