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 | 1 |
| 4 | -23 |
| 5 | -239 |
| 6 | -1439 |
| 7 | -6719 |
| 8 | 33601 |
| 9 | 1536193 |
| 10 | 24171841 |
| 11 | 268424641 |
| 12 | 1144566721 |
| 13 | -47515765439 |
| 14 | -1727426116415 |
| 15 | -36344982098879 |
| 16 | -481057514071679 |
| 17 | 1197767242412161 |
| 18 | 319851095455612801 |
| 19 | 12145632936380316289 |
| 20 | 293167011107091899521 |
| 21 | 3520557699737168603521 |