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 | 4 |
| 4 | -18 |
| 5 | 110 |
| 6 | -810 |
| 7 | 7040 |
| 8 | -70280 |
| 9 | 792200 |
| 10 | -9945000 |
| 11 | 137550400 |
| 12 | -2077719600 |
| 13 | 34026132400 |
| 14 | -600433397200 |
| 15 | 11356783360000 |
| 16 | -229193571984000 |
| 17 | 4915556301968000 |
| 18 | -111644265232656000 |
| 19 | 2676908220027392000 |