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) |
|---|---|
| 1 | 1 |
| 2 | -2 |
| 3 | 4 |
| 4 | -10 |
| 5 | 30 |
| 6 | -104 |
| 7 | 392 |
| 8 | -1568 |
| 9 | 6520 |
| 10 | -27976 |
| 11 | 122944 |
| 12 | -551680 |
| 13 | 2518912 |
| 14 | -11684000 |
| 15 | 54957216 |
| 16 | -261897024 |
| 17 | 1263216192 |
| 18 | -6164172608 |
| 19 | 30416619200 |
| 20 | -151750104800 |
| 21 | 765364073120 |
| 22 | -3902783995520 |
| 23 | 20123276097920 |