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 | -5 |
| 4 | 20 |
| 5 | -104 |
| 6 | 584 |
| 7 | -4304 |
| 8 | 35720 |
| 9 | -329160 |
| 10 | 3239112 |
| 11 | -36135912 |
| 12 | 438454752 |
| 13 | -5743527360 |
| 14 | 80351263680 |
| 15 | -1218698312064 |
| 16 | 19599583392384 |
| 17 | -334335747652224 |
| 18 | 6019295318075520 |
| 19 | -114911886106373760 |
| 20 | 2305234779285164544 |
| 21 | -48596575400453366784 |