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 | 25 |
| 5 | -241 |
| 6 | 1441 |
| 7 | -6721 |
| 8 | 67201 |
| 9 | -1185409 |
| 10 | 16652161 |
| 11 | -180639361 |
| 12 | 2098673281 |
| 13 | -37526586241 |
| 14 | 785718950017 |
| 15 | -14516030954881 |
| 16 | 247504017895681 |
| 17 | -4832929862019841 |
| 18 | 116556246644716801 |
| 19 | -2930255897793414913 |
| 20 | 69746855593499124481 |
| 21 | -1673960044278244020481 |