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 | -4 |
| 4 | 0 |
| 5 | 20 |
| 6 | 20 |
| 7 | -120 |
| 8 | -280 |
| 9 | 800 |
| 10 | 3600 |
| 11 | -5200 |
| 12 | -48400 |
| 13 | 19200 |
| 14 | 696800 |
| 15 | 408800 |
| 16 | -10740000 |
| 17 | -17689600 |
| 18 | 175630400 |
| 19 | 511732800 |
| 20 | -3000875200 |
| 21 | -13747264000 |
| 22 | 52271990400 |
| 23 | 368459062400 |
| 24 | -886068707200 |
| 25 | -10097545267200 |
| 26 | 12940241120000 |
| 27 | 285573963334400 |