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 | 3 |
| 4 | 4 |
| 5 | -90 |
| 6 | 786 |
| 7 | -5670 |
| 8 | 34784 |
| 9 | -136584 |
| 10 | -824760 |
| 11 | 33137280 |
| 12 | -633666648 |
| 13 | 10089623544 |
| 14 | -145675230960 |
| 15 | 1910939579640 |
| 16 | -21215723677440 |
| 17 | 136130901474240 |
| 18 | 2280768466608576 |
| 19 | -135531682778927808 |
| 20 | 4380044490023909760 |
| 21 | -119144344839822570240 |