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 | -3 |
| 4 | 9 |
| 5 | -27 |
| 6 | 81 |
| 7 | -243 |
| 8 | 729 |
| 9 | -2187 |
| 10 | 6561 |
| 11 | -19683 |
| 12 | 59049 |
| 13 | -177147 |
| 14 | 531441 |
| 15 | -1594323 |
| 16 | 4782969 |
| 17 | -14348907 |
| 18 | 43046721 |
| 19 | -129140163 |
| 20 | 387420489 |
| 21 | -1162261467 |
| 22 | 3486784401 |
| 23 | -10460353203 |
| 24 | 31381059609 |
| 25 | -94143178827 |
| 26 | 282429536481 |
| 27 | -847288609443 |