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 | 6 |
| 4 | 0 |
| 5 | -120 |
| 6 | 1440 |
| 7 | -10080 |
| 8 | 40320 |
| 9 | 362880 |
| 10 | -10886400 |
| 11 | 159667200 |
| 12 | -1437004800 |
| 13 | 0 |
| 14 | 348713164800 |
| 15 | -9153720576000 |
| 16 | 146459529216000 |
| 17 | -1067062284288000 |
| 18 | -25609494822912000 |
| 19 | 1338096104497152000 |
| 20 | -34060628114472960000 |
| 21 | 510909421717094400000 |