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 | -1 |
| 3 | -1 |
| 4 | 0 |
| 5 | 9 |
| 6 | -45 |
| 7 | 447 |
| 8 | -2744 |
| 9 | 17553 |
| 10 | -171585 |
| 11 | 1757535 |
| 12 | -19723176 |
| 13 | 245370969 |
| 14 | -3189613245 |
| 15 | 44636677407 |
| 16 | -674857335120 |
| 17 | 10851333193249 |
| 18 | -185485926579489 |
| 19 | 3356664148618047 |
| 20 | -64009236131219760 |
| 21 | 1284480775318317225 |