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 | 1 |
| 1 | 1 |
| 2 | -1 |
| 3 | 3 |
| 4 | -13 |
| 5 | 65 |
| 6 | -353 |
| 7 | 2025 |
| 8 | -12077 |
| 9 | 74143 |
| 10 | -465481 |
| 11 | 2974863 |
| 12 | -19289821 |
| 13 | 126594191 |
| 14 | -839273105 |
| 15 | 5612483619 |
| 16 | -37814455781 |
| 17 | 256447068841 |
| 18 | -1749182184793 |
| 19 | 11991887667273 |
| 20 | -82588248514885 |
| 21 | 571118483653841 |