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 | -2 |
| 2 | 6 |
| 3 | -20 |
| 4 | 50 |
| 5 | 168 |
| 6 | -4732 |
| 7 | 54024 |
| 8 | -356670 |
| 9 | -1558040 |
| 10 | 106069172 |
| 11 | -2197188864 |
| 12 | 26605890220 |
| 13 | 22266781600 |
| 14 | -12120090377400 |
| 15 | 402165029201744 |
| 16 | -7732409047854318 |
| 17 | 38209542402620232 |
| 18 | 4126723132306766900 |
| 19 | -211906442415502467360 |
| 20 | 5955735723962955009156 |