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 | 1 |
| 4 | -2 |
| 5 | 4 |
| 6 | -12 |
| 7 | 36 |
| 8 | -144 |
| 9 | 576 |
| 10 | -2880 |
| 11 | 14400 |
| 12 | -86400 |
| 13 | 518400 |
| 14 | -3628800 |
| 15 | 25401600 |
| 16 | -203212800 |
| 17 | 1625702400 |
| 18 | -14631321600 |
| 19 | 131681894400 |
| 20 | -1316818944000 |
| 21 | 13168189440000 |
| 22 | -144850083840000 |
| 23 | 1593350922240000 |
| 24 | -19120211066880000 |