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 | 0 |
| 3 | -6 |
| 4 | 18 |
| 5 | -90 |
| 6 | 660 |
| 7 | -3360 |
| 8 | 47880 |
| 9 | -293160 |
| 10 | 4277280 |
| 11 | -36424080 |
| 12 | 575190000 |
| 13 | -6745218480 |
| 14 | 101911249440 |
| 15 | -1628086299840 |
| 16 | 24861230634240 |
| 17 | -484979925830400 |
| 18 | 7629427896330240 |
| 19 | -176975913961566720 |
| 20 | 3036472694482106880 |
| 21 | -77953392499390087680 |