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 | -4 |
| 3 | 15 |
| 4 | 0 |
| 5 | -1155 |
| 6 | 20160 |
| 7 | -208845 |
| 8 | 0 |
| 9 | 68139225 |
| 10 | -2075673600 |
| 11 | 34976316375 |
| 12 | 0 |
| 13 | -25949801752875 |
| 14 | 1126343522304000 |
| 15 | -26264240610733125 |
| 16 | 0 |
| 17 | 34770736214117528625 |
| 18 | -1958486116582195200000 |
| 19 | 58318039100493206409375 |
| 20 | 0 |
| 21 | -120842042784862988395681875 |
| 22 | 8366746697372733839769600000 |