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 | 3 |
| 2 | 1 |
| 3 | -3 |
| 4 | 3 |
| 5 | -15 |
| 6 | 45 |
| 7 | -315 |
| 8 | 315 |
| 9 | -2835 |
| 10 | 14175 |
| 11 | -155925 |
| 12 | 467775 |
| 13 | -6081075 |
| 14 | 42567525 |
| 15 | -638512875 |
| 16 | 638512875 |
| 17 | -10854718875 |
| 18 | 97692469875 |
| 19 | -1856156927625 |
| 20 | 9280784638125 |
| 21 | -194896477400625 |
| 22 | 2143861251406875 |
| 23 | -49308808782358125 |
| 24 | 147926426347074375 |
| 25 | -3698160658676859375 |