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 | -2 |
| 3 | -6 |
| 4 | 40 |
| 5 | 120 |
| 6 | -1872 |
| 7 | -3920 |
| 8 | 155776 |
| 9 | 56448 |
| 10 | -19946240 |
| 11 | 44799744 |
| 12 | 3588719616 |
| 13 | -21265587200 |
| 14 | -850126505984 |
| 15 | 9423227873280 |
| 16 | 251457224998912 |
| 17 | -4665150579572736 |
| 18 | -88212028284665856 |
| 19 | 2663461772025462784 |
| 20 | 34353949630376181760 |
| 21 | -1756678038088484388864 |