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 | -5 |
| 5 | 63 |
| 6 | -315 |
| 7 | 99 |
| 8 | -675675 |
| 9 | 135135 |
| 10 | -1640925 |
| 11 | 83329155 |
| 12 | -4583103525 |
| 13 | 5791834125 |
| 14 | -71152682225625 |
| 15 | 14230536445125 |
| 16 | -26797763435625 |
| 17 | 24833960277501375 |
| 18 | -73881031825566590625 |
| 19 | 8546099690638125 |
| 20 | -106610328924292590271875 |