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 | -5 |
| 3 | 25 |
| 4 | -145 |
| 5 | 945 |
| 6 | -6641 |
| 7 | 49057 |
| 8 | -375361 |
| 9 | 2948353 |
| 10 | -23634049 |
| 11 | 192554753 |
| 12 | -1589812225 |
| 13 | 13272519937 |
| 14 | -111850866433 |
| 15 | 950220134913 |
| 16 | -8129133081601 |
| 17 | 69971682467841 |
| 18 | -605546841831425 |
| 19 | 5265763716550657 |
| 20 | -45988028107350017 |
| 21 | 403192288488677377 |