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 | -75 |
| 6 | 435 |
| 7 | -3045 |
| 8 | 29505 |
| 9 | -305865 |
| 10 | 3193785 |
| 11 | -36542835 |
| 12 | 501892335 |
| 13 | -7286043555 |
| 14 | 108056360475 |
| 15 | -1793622856725 |
| 16 | 32996752625025 |
| 17 | -620276725767825 |
| 18 | 11975531764755825 |
| 19 | -249801487147455075 |
| 20 | 5592450521376068175 |
| 21 | -130966142569782327675 |