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 | 3 |
| 4 | -15 |
| 5 | 105 |
| 6 | -945 |
| 7 | 10395 |
| 8 | -135135 |
| 9 | 2027025 |
| 10 | -34459425 |
| 11 | 654729075 |
| 12 | -13749310575 |
| 13 | 316234143225 |
| 14 | -7905853580625 |
| 15 | 213458046676875 |
| 16 | -6190283353629375 |
| 17 | 191898783962510625 |
| 18 | -6332659870762850625 |
| 19 | 221643095476699771875 |
| 20 | -8200794532637891559375 |
| 21 | 319830986772877770815625 |