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 | 2 |
| 2 | 1 |
| 3 | -1 |
| 4 | -3 |
| 5 | -1 |
| 6 | -7 |
| 7 | -1 |
| 8 | -43 |
| 9 | -1 |
| 10 | -1807 |
| 11 | -1 |
| 12 | -3263443 |
| 13 | -1 |
| 14 | -10650056950807 |
| 15 | -1 |
| 16 | -113423713055421844361000443 |
| 17 | -1 |
| 18 | -12864938683278671740537145998360961546653259485195807 |
| 19 | -1 |