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 | 0 |
| 2 | 2 |
| 3 | -3 |
| 4 | 20 |
| 5 | -90 |
| 6 | 594 |
| 7 | -4200 |
| 8 | 34544 |
| 9 | -316008 |
| 10 | 3207240 |
| 11 | -35699400 |
| 12 | 432690312 |
| 13 | -5672581200 |
| 14 | 79991160144 |
| 15 | -1207367605080 |
| 16 | 19423062612480 |
| 17 | -331770360922560 |
| 18 | 5997105160795584 |
| 19 | -114373526841360000 |
| 20 | 2295170834453089920 |
| 21 | -48344592370577247360 |