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 | 0 |
| 1 | 1 |
| 2 | -2 |
| 3 | 5 |
| 4 | -15 |
| 5 | 54 |
| 6 | -240 |
| 7 | 1350 |
| 8 | -9450 |
| 9 | 78120 |
| 10 | -725760 |
| 11 | 7371000 |
| 12 | -81081000 |
| 13 | 965487600 |
| 14 | -12454041600 |
| 15 | 173675502000 |
| 16 | -2605132530000 |
| 17 | 41763850128000 |
| 18 | -711374856192000 |
| 19 | 12817252047600000 |
| 20 | -243527788904400000 |