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 | 2 |
| 2 | -4 |
| 3 | 20 |
| 4 | -120 |
| 5 | 800 |
| 6 | -5696 |
| 7 | 42416 |
| 8 | -326304 |
| 9 | 2572992 |
| 10 | -20685696 |
| 11 | 168920704 |
| 12 | -1397257472 |
| 13 | 11682707712 |
| 14 | -98578346496 |
| 15 | 838369268480 |
| 16 | -7178912946688 |
| 17 | 61842549386240 |
| 18 | -535575159363584 |
| 19 | 4660216874719232 |
| 20 | -40722264390799360 |
| 21 | 357204260381327360 |